Control system for an internal combustion engine

ABSTRACT

A control system for an internal combustion engine including a direct cylinder fuel injection valve for directly injecting the fuel into a cylinder of an internal combustion engine. The main fuel that burns in the cylinder is injected in the latter half of the compression stroke of the cylinder, and the secondary fuel which is the ineffective fuel that does not burn in the cylinder is injected in the latter half of the exhaust stroke main fuel and the secondary fuel air-fuel ratio mixture around the spark plug. When the secondary fuel is injected, the exhaust valve is opening, and the fuel that is deflected is all discharged out of the cylinder through the exhaust port. Therefore, the ineffective fuel supplied by the secondary fuel injection does not remain in the cylinder, and the output torque does not change in the combustion of the next cycle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control system for an internalcombustion engine. More specifically, the invention relates to a controlsystem for an internal combustion engine, for supplying ineffective fuelwhich does not burn in the combustion chamber.

2. Description of the Related Art

There has been known a technology for adjusting the air-fuel ratio ofthe exhaust gas from an engine independently of the engine operatingair-fuel ratio (combustion air-fuel ratio in the combustion chamber) bysupplying, to the engine, ineffective fuel that does not contribute tothe combustion in the combustion chamber. For example, a NO_(x)occluding and reducing catalyst is disposed in the exhaust passage ofthe engine which operates at a lean air-fuel ratio, the NO_(x) occludingand reducing catalyst absorbing NO_(x) in the exhaust gas when theair-fuel ratio of the exhaust gas flowing in is lean, and releasing andpurifying by reduction the absorbed NO_(x) when the air-fuel ratio inthe exhaust gas becomes rich. In this case, the air-fuel ratio of theexhaust gas flowing into the NO_(x) occluding and reducing catalyst mustbe set to be rich at regular intervals and the NO_(x) must be releasedfrom the NO_(x) occluding and reducing catalyst, so that the NO_(x)occluding and reducing catalyst will not be saturated with NO_(x) whenthe engine is operated at a lean air-fuel ratio. In such a case, achange in the engine operating air-fuel ratio from the lean side to therich side increases the engine output torque; i.e., a change in theair-fuel ratio changes the torque. Upon supplying ineffective fuel thatdoes not contribute to the combustion in the engine combustion chamber,i.e., that does not burn in the engine combustion chamber, therefore, itbecomes possible to advantageously change only the air-fuel ratio of theexhaust gas independently from the engine operating air-fuel ratio. Inan engine having direct cylinder fuel injection valves for directlyinjecting the fuel into the cylinders, the ineffective fuel can besupplied into the cylinders by secondary fuel injection in the expansionstroke or in the exhaust stroke of the cylinders. In an engine havingexhaust port fuel injection valves for injecting the fuel into theexhaust port of the engine, further, the ineffective fuel can besupplied into the exhaust ports by the exhaust port fuel injection.

The fuel injected into the cylinder during the expansion stroke or theexhaust stroke or the fuel injected into the exhaust port of thecylinder is vaporized without being burned and is discharged togetherwith the exhaust gas. That is, the ineffective fuel that is supplieddoes not contribute to the combustion in the engine, but the amount ofthe unburned HC component in the exhaust gas from the engine increasesby an amount of the ineffective fuel that is supplied to establish arich air-fuel ratio. By supplying the ineffective fuel to the engine,therefore, it is possible to change the air-fuel ratio only in theexhaust gas from the engine without affecting the engine operatingair-fuel ratio.

A device for supplying the ineffective fuel of this type has beendisclosed in, for example, Japanese Unexamined Patent Publication(Kokai) No. 6-212961.

According to the device of this publication, a NO_(x) occluding andreducing catalyst is disposed in the exhaust passage of a diesel engineto absorb the NO_(x) in the exhaust gas when the air-fuel ratio of theexhaust gas flowing in is lean and to release the NO_(x) when the oxygenconcentration has decreased in the exhaust gas that is flowing in. Undernormal condition, the main fuel is injected into the cylinder near thecompressive top dead center of the cylinder of the engine and, when theNO_(x) is to be released from the NO_(x) occluding and reducingcatalyst, the secondary fuel is injected during the expansion stroke orthe exhaust stroke of the engine in addition to injecting the main fuel.The fuel injected into the cylinder during the expansion or exhauststroke does not contribute to the combustion in the cylinder, i.e., doesnot burn in the cylinder and is exposed to the burned gas of a hightemperature in the cylinder. Therefore, hydrocarbons having largemolecular weights in the fuel are decomposed into hydrocarbons havingsmall molecular weights. Besides, the fuel supplied by the secondaryfuel injection does not contribute to the combustion but is simplydischarged from cylinders together with the exhaust gas. By supplyingthe ineffective fuel to the engine, therefore, it is made possible toinject the fuel in a relatively large amount for establishing a richair-fuel ratio in the exhaust gas without increasing the pressure of anexplosion in the cylinder even in a diesel engine. According to thedevice of the above-mentioned publication, when the secondary fuel isinjected, the exhaust gas having a rich air-fuel ratio, containing alarge amount of hydrocarbons of low molecular weights which are highlyactive, flows into the NO_(x) occluding and reducing catalyst in theexhaust passage. When the secondary fuel is injected, therefore, theNO_(x) that has been absorbed is released from the NO_(x) occluding andreducing catalyst and is purified by reduction with hydrocarbons in theexhaust gas.

In an engine which effects the secondary fuel injection as done by thedevice of the above-mentioned publication, however, the fuel supplied bythe secondary fuel injection is not completely exhausted during theexhaust stroke but often remains in the cylinder. When the fuel of thesecondary fuel injection partly remains in the cylinder, this remainingfuel burns in the cylinder in addition to the fuel supplied by the mainfuel injection at the time when the main fuel is injected next time.Accordingly, an amount of fuel burnt in the engine increases, whereby anincreased torque is produced by the combustion. This causes a change inthe output torque of the engine.

When the ineffective fuel is supplied to the engine by the exhaust portfuel injection without relying upon the secondary fuel injection, on theother hand, the fuel does not remain in the cylinder. In an engineequipped with an exhaust gas recirculation (EGR) device, however, thesimilar problem may occur when the exhaust port fuel injection iseffected.

There has been generally known an exhaust gas recirculation (EGR) devicein which the exhaust gas from the engine is partly recirculated into thecombustion chamber of an internal combustion engine to lower thecombustion temperature in the combustion chamber in order to decreasethe amount of NO_(x) (nitrogen oxides) formed by the combustion. Theexhaust gas recirculation system includes an external EGR system inwhich an exhaust passage of the engine is connected to an intake passageof the engine through an. EGR passage, and the amount of the exhaust gasto be recirculated is adjusted by a flow rate adjusting valve (EGRvalve) provided in the EGR passage, and an internal EGR system by whichthe amount of blow back of the burned gas in the combustion chambercaused by the overlapping of valve is adjusted by changing theopen-close timings of the intake valve and the exhaust valve of theengine.

When the ineffective fuel is supplied to the engine that utilizes theEGR (exhaust gas recirculation) as described above, there a problemoccurs not only when the ineffective fuel is supplied by the secondaryfuel injection but also when the ineffective fuel is supplied by theexhaust port injection.

That is, when the ineffective fuel is supplied as described above, theexhaust gas from the engine contains unburned fuel in relatively largeamounts. When the exhaust gas is directly recirculated by the EGR deviceinto the combustion chamber of the engine, part of the ineffective fuelthat should not burn in the combustion chamber is recirculated into thecombustion chamber and burns therein. When the ineffective fuel issupplied while the EGR is being performed, therefore, the fuel issupplied in an excess amount into the engine and the combustion air-fuelratio becomes excessively rich, whereby the combustion in the combustionchamber becomes unstable or the output torque of the engine increasesdue to the combustion of excess fuel.

SUMMARY OF THE INVENTION

In view of the problems in the related art as set forth above, theobject of the present invention is to provide a control system for aninternal combustion engine capable of preventing the combustion frombecoming unstable and preventing a change in the output torque caused bythe residual fuel or recirculation of the fuel in the combustionchamber, when the fuel is supplied to the engine as ineffective fuel.

The object as set forth above is achieved by a control system for aninternal combustion engine according to the present invention,comprising:

a direct cylinder fuel injection valve for directly injecting the fuelinto a cylinder of an internal combustion engine; and

a fuel injection control means which executes a main fuel injection toinject the fuel that burns in the cylinder by controlling the directcylinder fuel injection valve, and further executes, as required, asecondary fuel injection to inject the fuel that does not burn in thecylinder during the expansion stroke or the exhaust stroke after themain fuel injection; wherein

when the secondary fuel injection is being executed, the fuel injectioncontrol means controls the secondary fuel injection based on the engineoperating conditions in such a manner that the fuel supplied by thesecondary fuel injection is exhausted out of the cylinder before the endof the exhaust stroke.

According to this aspect of the invention, the fuel injection controlmeans controls the secondary fuel injection based on the engineoperating conditions such as valve timing, rotational speed, etc., andchanges, for example, the amount of fuel injection, injection timing,etc., so that the fuel supplied by the secondary fuel injection is alldischarged out of the cylinder before the end of the exhaust stroke,i.e., before the exhaust valve is closed. Thus, there remains no fuel inthe cylinder, and the engine output torque does not change even when thesecondary fuel is injected.

According to another aspect of the present invention, there is provideda control system for an internal combustion engine comprising:

a direct cylinder fuel injection valve for directly injecting the fuelinto a cylinder of an internal combustion engine; and

a fuel injection control means which executes a main fuel injection toinject the fuel that burns in the cylinder by controlling the directcylinder fuel injection valve, and executes, as required, a secondaryfuel injection to inject the fuel that does not burn in the cylinderduring the expansion stroke or the exhaust stroke after the main fuelinjection; wherein

the fuel injection control means advances the timing for injecting thesecondary fuel with an increase in the amount of the fuel injected bythe secondary fuel injection.

According to this aspect of the invention, the fuel injection controlmeans advances the timing for injecting the secondary fuel with anincrease in the amount of the secondary fuel injection. Therefore, evenwhen the amount of the secondary fuel injection is large, a sufficientperiod of time is maintained before the exhaust valve is closed, and theinjected fuel does not remain in the cylinder. Accordingly, the engineoutput torque does not change even when the secondary fuel is injected.

According to a further aspect of the present invention, there isprovided a control system for an internal combustion engine comprising:

a direct cylinder fuel injection valve for directly injecting the fuelinto a cylinder of an internal combustion engine;

a fuel injection control means which executes a main fuel injection toinject the fuel that burns in the cylinder by controlling the directcylinder fuel injection valve, and executes, as required, a secondaryfuel injection to inject the fuel that does not burn in the cylinderduring the expansion stroke or the exhaust stroke after the main fuelinjection; and

a deflecting means for deflecting the flow of the fuel supplied by thesecondary fuel injection toward the exhaust port of the cylinder.

According to this aspect of the invention, the deflecting means deflectsthe fuel supplied by the secondary fuel injection toward the exhaustport. Therefore, the whole amount of the fuel supplied by the secondaryfuel injection is discharged from the exhaust port and does not remainin the cylinder. Therefore, the engine output torque does not changeeven when the secondary fuel is injected.

According to a further aspect of the present invention, there isprovided a control system for an internal combustion engine comprising:

a direct cylinder fuel injection valve for directly injecting the fuelinto a cylinder of an internal combustion engine; and

a fuel injection control means which executes a main fuel injection toinject the fuel that burns in the cylinder by controlling the directcylinder fuel injection valve, and executes, as required, a secondaryfuel injection to inject the fuel that does not burn in the cylinderduring the expansion stroke or the exhaust stroke after the main fuelinjection; wherein

the fuel injection control means sets the pressure of the secondary fuelinjection to be lower than the pressure of the main fuel injection.

According to this aspect of the invention, the fuel injection controlmeans sets the pressure of the secondary fuel injection to be lower thanthe pressure of the main fuel injection. Therefore, the fuel supplied bythe secondary fuel injection does not collide with the cylinder wall orthe piston to adhere thereon, and is discharged together with theexhaust gas from the cylinder. Therefore, no fuel remains in thecylinder, and the engine output torque does not change even when thesecondary fuel is injected.

According to a further aspect of the present invention, there isprovided a control system for an internal combustion engine comprising:

a direct cylinder fuel injection valve for directly injecting the fuelinto a cylinder of an internal combustion engine; and

a fuel injection control means which executes a main fuel injection toinject the fuel that burns in the cylinder by controlling the directcylinder fuel injection valve, and executes, as required, a secondaryfuel injection to inject the fuel that does not burn in the cylinderduring the expansion stroke or the exhaust stroke after the main fuelinjection; wherein

the fuel injection control means calculates the amount of fuel remainingin the cylinder, which is part of the fuel supplied by the precedingsecondary fuel injection, and corrects the amount of the main fuelinjection based on the remaining amount of fuel.

According to this aspect of the present invention, the fuel injectioncontrol means calculates the amount of fuel remaining in the cylinderdue to the preceding secondary fuel injection, and corrects the amountof the main fuel injection depending upon the remaining amount of fuel.This correction is effected by, for example, decreasing the amount ofthe main fuel injection by an amount corresponding to the remainingamount of fuel. Therefore, the amount of fuel that contributes to thecombustion at the time when the main fuel is injected is maintained at atarget amount. Therefore, the engine output torque does not change evenwhen the fuel remains in the cylinder due to the secondary fuelinjection.

According to a further aspect of the present invention, there isprovided a control system for an internal combustion engine comprising:

a direct cylinder fuel injection valve for directly injecting the fuelinto a cylinder of an internal combustion engine; and

a fuel injection control means which executes a main fuel injection toinject the fuel that burns in the cylinder by controlling the directcylinder fuel injection valve, and executes, as required, a secondaryfuel injection to inject the fuel that does not burn in the cylinderduring the expansion stroke or the exhaust stroke after the main fuelinjection; wherein

the fuel injection control means, as required, executes the main fuelinjection two times by dividing it into a first main fuel injection forforming a uniform air-fuel mixture in the cylinder and a second mainfuel injection for forming a charge of a combustible air-fuel ratiomixture in the cylinder, and, when the secondary fuel injection isexecuted, the fuel injection means calculates the amount of fuelremaining in the cylinder, which is part of the fuel supplied by thepreceding secondary fuel injection, and corrects the amount of the firstmain fuel injection based on the remaining amount of fuel.

According to this aspect of the present invention, the fuel injectioncontrol means, as required, executes the main fuel injection twice and,in this case, corrects the amount of the first main fuel injection basedon the amount of fuel remaining in the cylinder due to the secondaryfuel injection. This correction is effected by, for example, decreasingthe amount of the first fuel injection by an amount corresponding to theremaining amount of fuel. The first main fuel injection is for forming auniform mixture in the cylinder whereas the second main fuel injectionis for forming a stratified charge of the mixture. On the other hand,the fuel remaining in the cylinder forms a uniform mixture in thecylinder. When the amount of the first main fuel injection is normallyset, therefore, the air-fuel ratio of the formed uniform mixture becomesmore rich than the target value. According to the present invention,therefore, the amount of the first main fuel injection is correctedbased on the remaining amount of fuel, so that the air-fuel ratio of theuniform mixture formed in the cylinder is maintained at the targetvalue.

According to a further aspect of the present invention, there isprovided a control system for an internal combustion engine comprising:

an ineffective fuel supply means for supplying ineffective fuel thatdoes not burn in the combustion chamber of an internal combustionengine;

an EGR means for recirculating the exhaust gas from the engine into thecombustion chamber of the engine; and

an EGR limiting means for limiting the exhaust gas recirculated by theEGR means when the ineffective fuel is being supplied to the engine bythe ineffective fuel supply means.

According to this aspect of the present invention, the EGR is limited bythe EGR limiting means when the ineffective fuel is being supplied.Therefore, the ineffective fuel is recirculated in a decreased amountinto the combustion chamber of the engine together with the recirculatedexhaust gas, making it possible to prevent the combustion in thecombustion chamber from becoming unstable and to prevent a change in theoutput torque. Here, “limit the recirculation of exhaust gas” includesboth the case where the exhaust gas is recirculated in a decreasedamount and the case where the recirculation of the exhaust gas iscompletely interrupted.

According to a further aspect of the present invention, there isprovided a control system for an internal combustion engine comprising:

an ineffective fuel supply means for supplying ineffective fuel thatdoes not burn in the combustion chamber of an internal combustionengine;

an EGR means for recirculating the exhaust gas from the engine into thecombustion chamber of the engine; and

an ineffective fuel limiting means for limiting the supply of theineffective fuel by the ineffective fuel supply means when the exhaustgas is recirculated by the EGR means.

According to this aspect of the present invention, the supply of theineffective fuel is limited by the ineffective fuel limiting means whenthe EGR is executed. Therefore, the ineffective fuel is recirculatedinto the combustion chamber of the engine together with the recirculatedexhaust gas. Here, “limit the supply of the ineffective fuel” includesboth the case where the ineffective fuel is supplied in a decreasedamount and the case where the supply of the ineffective fuel iscompletely interrupted.

According to a further aspect of the present invention, there isprovided a control system for an internal combustion engine comprising:

a main fuel supply means for supplying, into the engine, the fuel thatburns in the combustion chamber based on the operating conditions of theinternal combustion engine;

an ineffective fuel supply means for supplying, into the engine, theineffective fuel that does not burn in the combustion chamber of theengine;

an EGR means for recirculating the exhaust gas of the engine into thecombustion chamber of the engine; and

a correction means for estimating the amount of the ineffective fuel inthe exhaust gas recirculated by the EGR means to correct the amount offuel supplied to the engine by the main fuel supply means based on theestimated amount.

According to this aspect of the present invention, the amount of themain fuel is corrected depending upon the amount of the ineffective fuelthat recirculates into the combustion chamber together with the exhaustgas. Therefore, the fuel is supplied in a proper amount into thecombustion chamber based on the engine operating conditions irrespectiveof the recirculating amount of the ineffective fuel, preventing thecombustion in the combustion chamber from losing stability andpreventing a change in the output torque.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the description asset forth hereinafter with reference to the accompanying drawings inwhich:

FIG. 1 is a diagram schematically illustrating the general constructionof an embodiment when the present invention is applied to an internalcombustion engine for automobiles;

FIG. 2 is a vertical sectional view of a cylinder for illustrating afirst embodiment of the present invention;

FIG. 3 is a flow chart illustrating the operation for controlling thesecondary fuel injection according to the first embodiment;

FIG. 4 is a vertical sectional view of a cylinder for illustrating asecond embodiment of the present invention;

FIG. 5 is a flow chart illustrating the operation for controlling thesecondary fuel injection according to the second embodiment;

FIG. 6 is a diagram similar to FIG. 4 and illustrates a modified exampleof the second embodiment;

FIG. 7 is a diagram illustrating a method of calculating the remainingamount of fuel according to a third embodiment of the present invention;

FIG. 8 is a flow chart illustrating the operation for controlling themain fuel injection according to the third embodiment;

FIG. 9 is a diagram schematically illustrating the general constructionof an embodiment when the present invention is applied to an internalcombustion engine that executes the external EGR for automobiles;

FIG. 10 is a flow chart illustrating the secondary fuel injectionoperation according to a fourth embodiment of the present invention;

FIG. 11 is a diagram schematically illustrating the general constructionof an embodiment when the present invention is applied to an internalcombustion engine that executes the internal EGR for automobiles;

FIG. 12 is a diagram illustrating valve timings of the engine of FIG.11;

FIG. 13 is a flow chart illustrating the secondary fuel injectionoperation according to a sixth embodiment of the present invention;

FIG. 14 is a flow chart illustrating the secondary fuel injectionoperation according to the sixth embodiment of the present invention butis different from that of FIG. 13;

FIG. 15 is a flow chart illustrating the secondary fuel injectionoperation according to a seventh embodiment of the present invention;

FIG. 16 is a diagram illustrating a method of calculating the amount ofunburned fuel recirculated into the combustion chamber; and

FIG. 17 is a flow chart illustrating the operation for correcting theamount of main fuel injection according to an eighth embodiment of thepresent invention using the method of FIG. 16.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the control system according to the presentinvention will be explained with reference to the attached drawings.

FIG. 1 is a view schematically illustrating the constitution of anembodiment in which a fuel injection device of the present invention isapplied to an internal combustion engine for an automobile.

In FIG. 1, reference numeral 1 denotes an internal combustion engine foran automobile. In this embodiment, the engine 1 is a four-cylindergasoline engine having four cylinders #1 to #4 which are equipped withfuel injection valves 111 to 114 for directly injecting fuel into thecylinders. As will be described later, the internal combustion engine 1of this embodiment is a lean-burn engine that can be operated at anair-fuel ratio higher (more lean) than the stoichiometric air-fuelratio.

In this embodiment, further, the cylinders #1 to #4 are grouped into twogroups of cylinders each group including two cylinders so that theignition timings will not take place consecutively (in the embodiment ofFIG. 1, for example, the order of igniting the cylinders is 1-3-4-2, thecylinders #1 and #4 constituting one group of cylinders, and thecylinders #2 and #3 constituting another group of cylinders). Theexhaust port of each cylinder is connected to an exhaust manifold ofeach group of cylinders, and is connected to an exhaust passage of eachgroup of cylinders. In FIG. 1, reference numeral 21 a denotes an exhaustmanifold for connecting exhaust ports of the group of the cylinders #1and #4 to an independent exhaust passage 2 a, and reference numeral 21 bdenotes an exhaust manifold for connecting exhaust ports of the group ofthe cylinders #2 and #4 to an independent exhaust passage 2 b. In thisembodiment, start catalysts (hereinafter referred to as “SCs”) 5 a and 5b comprising a three-way catalyst are arranged in the independentexhaust passages 2 a and 2 b. The independent exhaust passages 2 a and 2b meet together in a common exhaust passage 2 on the downstream side ofthe SCs.

An NO_(x) occluding and reducing catalyst 7 that will be described lateris arranged in the common exhaust passage 2. In FIG. 1, referencenumerals 29 a and 29 b denote air-fuel ratio sensors arranged on theupstream side of the start catalysts 5 a and 5 b of the independentexhaust passages 2 a and 2 b, and reference numeral 31 denotes anair-fuel ratio sensor arranged at an outlet of the NO_(x) occluding andreducing catalyst 7 in the exhaust passage 2. The air-fuel ratio sensors29 a, 29 b and 31 are so-called linear air-fuel ratio sensors thatproduce voltage signals corresponding to the air-fuel ratio of theexhaust gas over a wide range of air-fuel ratios.

In FIG. 1, further, intake ports of the cylinders #1 to #4 of the engine1 are connected to a surge tank 10 a through the intake manifold 10 b,the surge tank 10 a being connected to a common intake passage 10. Inthis embodiment, further, a throttle valve 15 is installed in the intakepassage 10. The throttle valve 15 in this embodiment is a so-calledelectronically controlled throttle valve which is driven by an actuator15 a of a suitable form such as a step motor to define a degree ofopening based on a control signal from an ECU 30 that will be describedlater. In FIG. 1, further, reference numeral 15 b denotes a throttlevalve opening-degree sensor (throttle sensor) for detecting the openingdegree of the throttle valve 15.

In this embodiment, the direct cylinder fuel injection valves 111 to 114are separately connected to a reservoir (common rail) 110 to inject thefuel of a high pressure in the common rail 110 into the cylinders. InFIG. 1, reference numeral 130 denotes a fuel pump comprising ahigh-pressure pump such as plunger pump. The fuel pump 130 supplies ahigh pressure fuel to the common rail 110 at a timing just after thefuel is injected by the fuel injection valves (111 to 114).

In FIG. 1, reference numeral 200 denotes a variable valve timing devicefor varying the valve timings of the engine 1. In this embodiment, thevariable valve timing device 200 may be any known type provided it iscapable of changing the valve timings of the engine based on aninstruction signal from an ECU 30 that will be described later, and maybe either one which changes the open-close timings only of the intakevalves and/or the exhaust valves, or one which changes the valve lift inaddition to the open-close timings. The valve timings may be changedeither continuously or stepwisely.

In FIG. 1, reference numeral 30 denotes the ECU (engine control unit)for controlling the engine 1. The ECU 30 comprises a widely knownmicrocomputer having RAM, ROM and CPU that are connected togetherthrough a bidirectional bus, and executes basic control operations suchas controlling the main fuel injection and the ignition timings. In thisembodiment, the ECU 30 further works to change the combustion in thecylinder into a rich air-fuel ratio during a regenerating operation ofthe NO_(x) occluding and reducing catalyst that will be described later,and controls the secondary fuel injection by injecting the secondaryfuel during the expansion or exhaust stroke of each cylinder to changethe air-fuel ratio of the exhaust gas flowing into the NO_(x) occludingand reducing catalyst to a rich air-fuel ratio within a short period oftime.

The input port of the ECU 30 receives signals from the air-fuel ratiosensors 29 a and 29 b representing the exhaust gas air-fuel ratios atthe inlet of the start catalysts 5 a and 5 b, a signal from the air-fuelratio sensor 31 representing an exhaust gas air-fuel ratio at the outletof the NO_(x) occluding and reducing catalyst 7, a signal correspondingto the intake air pressure of the engine from an intake-air-pressuresensor 37 provided in the surge tank 10 a, and a signal from theaccelerator opening-degree sensor 33 representing the amount of theaccelerator pedal depressed by the driver (accelerator opening degree),and a pulse signal from a rotational speed sensor 35 disposed near thecrankshaft (not shown) of the engine after every predeterminedrotational angle of the engine crankshaft. The ECU 30 calculates therotational angle of the crankshaft from the pulse signal, and calculatesthe rotational speed of the engine from the frequency of the pulsesignals. Further, the input port of the ECU 30 receives a signal from afuel pressure sensor 120 arranged in the common rail 110 representingthe fuel pressure in the common rail 110, and a signal from the throttlevalve opening-degree sensor 15 b representing the opening degree of thethrottle valve 15.

In order to control the amounts of fuel injection into the cylinders andto control the fuel injection timings, further, the output port of theECU 30 is connected to fuel injection valves 111 to 114 of the cylindersthrough a fuel injection circuit (not shown), and is further connectedto the actuator 15 b of the throttle valve 15 through a drive circuit(not shown) to control the opening degree of the throttle valve 15.

In addition to the above-mentioned control operations, the ECU 30controls by feedback the rate of the fuel supplied by the fuel pump 130based on the signal representing the fuel pressure in the common rail110 input from the fuel pressure sensor 120, so that the fuel pressurein the common rail is adjusted to a target value. The fuel is suppliedfrom the fuel pump 130 to the common rail 110 at a timing just after thefuel is injected by the fuel injection valves 111 to 114.

The output port of the ECU 30 is connected to the variable valve timingdevice 200 through a drive circuit (not shown) to control the valvetimings of the engine 1 based on the engine load conditions (degree ofaccelerator opening, engine rotational speed).

In this embodiment, the main fuel injection of the engine 1, i.e., theinjection of fuel for combustion in the cylinder, is controlled in thefollowing five modes based upon the loads exerted on the engine:

{circle around (1)} A lean air-fuel ratio stratified charge combustion(fuel is injected in the compression stroke).

{circle around (2)} A lean air-fuel ratio uniform mixture/stratifiedcharge combustion (fuel is injected in the suction stroke and in thecompression stroke).

{circle around (3)} A lean air-fuel ratio uniform mixture/stratifiedcharge combustion (fuel is injected in the suction stroke).

{circle around (4)} A stoichiometric air-fuel ratio uniform mixturecombustion (fuel is injected in the suction stroke).

{circle around (5)} A rich air-fuel ratio uniform mixture combustion(fuel is injected in the suction stroke).

That is, the lean air-fuel ratio stratified charge combustion {circlearound (1)} is carried out in the light-load operating region of theengine 1. In this state, the fuel is injected into the cylinders onlyone time in the latter half of the compression stroke in each cylinder,and the injected fuel forms a charge of a combustible mixture near thespark plug in the cylinder. In this operating state, further, the amountof fuel injected is very small, and the air-fuel ratio in the cylinderas a whole becomes from about 25 to about 30.

As the load increases from the above-mentioned state {circle around (1)}to enter into the low-load operation region, there takes place theabove-mentioned lean air-fuel ratio uniform mixture/stratified chargecombustion {circle around (2)}. The amount of fuel injected into thecylinder increases with an increase in the load exerted on the engine.In the above-mentioned stratified charge combustion {circle around (1)},the fuel is injected in the latter half of the compression stroke,whereby the injection time is limited and limitation is imposed on theamount of fuel for forming the stratified charge. In this load region,therefore, the fuel is injected in advance in the former half of thesuction stroke in an amount to compensate for the shortage of the fuelinjected in the latter half of the compression stroke, thereby to supplythe fuel in a target amount into the cylinder. The fuel injected intothe cylinder in the former half of the suction stroke forms a very leanand uniform mixture before being ignited. In the latter half of thecompression stroke, the fuel is further injected into this very lean anduniform mixture in order to form the charge of an ignitable andcombustible mixture near the spark plug. At the time of ignition, thiscombustible mixture charge starts burning, and the flame propagates tothe surrounding lean mixture charge, so that the combustion takes placestably. In this state, the amount of fuel injected in the suction strokeand in the compression stroke is larger than that of the mode {circlearound (1)}, but the air-fuel ratio as a whole is still lean (e.g.,air-fuel ratio of about 20 to about 30).

When the load on the engine further increases, the engine 1 is operatedin the lean air-fuel ratio uniform mixture combustion {circle around(3)}. In this state, the fuel is injected only one time in the formerhalf of the suction stroke, and the amount of the injected fuel becomeslarger than that of the mode {circle around (2)}. The uniform air-fuelmixture formed in the cylinder in this state has a lean air-fuel ratio(e.g., air-fuel ratio of from about 15 to about 25) relatively close tothe stoichiometric air-fuel ratio.

As the load on the engine further increases to enter into the high-loadoperation region of the engine, the amount of fuel becomes larger thanthat of the mode {circle around (3)}, and the engine is operated in thestoichiometric air-fuel ratio uniform mixture operation {circle around(4)}. In this state, a uniform mixture of the stoichiometric air-fuelratio is formed in the cylinder, and the engine output increases. Whenthe load on the engine further increases to enter into the full-loadoperation of the engine, the amount of fuel injection is furtherincreased in excess of that of the mode {circle around (4)}, and theengine is operated in the rich air-fuel ratio uniform mixture operation{circle around (5)}. In this state, the uniform mixture formed in thecylinder becomes a rich air-fuel ratio (e.g., air-fuel ratio of fromabout 12 to about 14).

In this embodiment, optimum operation modes {circle around (1)} to{circle around (5)} have been empirically set based upon the degree ofaccelerator opening (amount of the accelerator pedal depressed by thedriver) and the rotational speed of the engine, and a map using thedegree of accelerator opening and the engine rotational speed is storedin the ROM of the ECU 30. When the engine 1 is in operation, the ECU 30determines which one of the above-mentioned operation modes {circlearound (1)} to {circle around (5)} be selected based on the degree ofaccelerator opening detected by the accelerator opening-degree sensor 37and the rotational speed of the engine, and determines the amount offuel injection, timing for fuel injection, the number of times ofinjection and the degree of throttle valve opening based on each of themodes.

When the any one of the modes {circle around (1)} to {circle around (3)}(lean air-fuel ratio combustion) is selected, the ECU 30 determines theamount of fuel injection from the degree of accelerator opening and therotational speed of the engine based on a map that has been prepared inadvance for each of the modes {circle around (1)} to {circle around(3)}. When the mode {circle around (4)} or {circle around (5)}(stoichiometric air-fuel ratio or rich air-fuel ratio uniform mixturecombustion) is selected, the ECU 30 sets the amount of fuel injectionbased on the intake air pressure detected by the intake air pressuresensor 37 and the rotational speed of the engine by using a map that hasbeen prepared in advance for each of the modes {circle around (4)} and{circle around (5)}.

When the mode {circle around (4)} (stoichiometric air-fuel ratio uniformmixture combustion) is selected, the ECU 30 controls the air-fuel ratioby correcting the amount of fuel injection calculated above by feedbackbased on the outputs of the air-fuel ratio sensors 29 a and 29 b, sothat the air-fuel ratio of the exhaust gas emitted by the engine becomesthe stoichiometric air-fuel ratio.

The start catalysts (SCs) 5 a and 5 b are constituted as a three-waycatalyst by using a honeycomb-shaped substrate of cordierite or thelike, forming a thin coating of alumina on the surface of the substrate,and carrying a novel metal catalyst component such as platinum Pt,palladium Pd or rhodium Rh on the alumina layer. The three-way catalysthighly efficiently removes the three components, i.e., HC, CO and NO_(x)near the stoichiometric air-fuel ratio. The three-way catalyst exhibitsa decreased ability for reducing NO_(x) when the air-fuel ratio of theexhaust gas flowing in becomes higher than the stoichiometric air ratio.When the engine 1 is operating at a lean air-fuel ratio, therefore, thethree-way catalyst is not capable of removing NO_(x) in the exhaust gasto a sufficient degree.

In this embodiment, the start catalysts (SCs) 5 a and 5 b chiefly workto purify the exhaust gas of when the engine 1 is operating at a richair-fuel ratio immediately after the cold starting and to purity theexhaust gas of when the engine 1 is operating at the stoichiometricair-fuel ratio under normal operating condition. Therefore, the startcatalysts (SCs) 5 a and 5 b are disposed in the exhaust passages 2 a and2 b at positions close to the engine 1 and have a relatively smallcapacity to decrease their heat capacity, so that they can be heated totheir activated temperature within a short period of time after thestart of the engine to start their catalytic activity.

Next, described below is the NO_(x) occluding and reducing catalyst 7according to this embodiment. The NO_(x) occluding and reducing catalyst7 according to this embodiment uses alumina as a substrate to carry atleast one component selected from the alkali metals such as potassium K,sodium Na, lithium Li and cesium Cs, alkaline earth elements such asbarium Ba and calcium Ca, and rare earth elements such as lanthanum La,cerium Ce and yttrium Y, as well as a noble metal such as platinum Pt.The NO_(x) occluding and reducing catalyst exhibits the action ofabsorbing and releasing NO_(x), i.e., absorbs NO_(x) (nitrogen oxides)in the exhaust gas in the form of nitric acid ions NO₃ ⁻ when theair-fuel ratio of the exhaust gas flowing in is lean, and releases theabsorbed NO_(x) when the air-fuel ratio of the exhaust gas flowing inbecomes smaller than the stoichiometric air-fuel ratio (rich air-fuelratio).

The mechanism for absorbing and releasing NO_(x) will be described nextwith reference to the case of using platinum Pt and barium Ba. The samemechanism, however, is created even when other noble metals, alkalimetals, alkaline earth elements and rare earth elements are used.

When the concentration of oxygen increases in the exhaust gas that isflowing in (i.e., when the air-fuel ratio of the exhaust gas turns intoa lean air-fuel ratio), oxygen adheres in the form of O₂ ⁻ or O²⁻ ontoplatinum Pt, whereby NO_(x) in the exhaust gas reacts with O₂ ⁻ or O²⁻on platinum Pt thereby to form NO₂. NO₂ in the exhaust gas and NO₂ thusformed are further oxidized on platinum Pt, absorbed by the absorbingagent in which they are bonded to barium oxide BaO and are diffused inthe form of nitric acid ions NO₃ ⁻ in the absorbing agent. In a leanatmosphere, therefore, NO_(x) in the exhaust gas is absorbed in the formof nitrates by the NO_(x) absorbing agent.

When the concentration of oxygen greatly decreases in the exhaust gasthat is flowing in (i.e., when the air-fuel ratio of the exhaust gasbecomes smaller (more rich) than the stoichiometric air-fuel ratio), NO₂forms in a decreased amount on platinum Pt, and the reaction proceeds inthe reverse direction permitting nitric acid ions NO₃ ⁻ in the absorbingagent to be released in the form of NO₂ from the absorbing agent. Inthis case, the reducing components such as CO and the like and thecomponents such as HC, CO₂ and the like in the exhaust gas work toreduce NO₂ on platinum Pt.

In this embodiment, the engine 1 is normally operated at a lean air-fuelratio in most of the load regions except the high-load operation, andthe NO_(x) occluding and reducing catalyst absorbs NO_(x) in the exhaustgas that flows in. When the engine 1 is operated at a rich air-fuelratio, the NO_(x) occluding and reducing catalyst 7 releases andpurifies the absorbed NO_(x) by reduction. When the NO_(x) is absorbedin increased amounts by the NO_(x) occluding and reducing catalyst 7during the operation at a lean air-fuel ratio, therefore, a rich-spikeoperation is carried out to change the air-fuel ratio of the engine froma lean air-fuel ratio to a rich air-fuel ratio for a short period oftime in order to release NO_(x) from the NO_(x) occluding and reducingcatalyst and to purify NO_(x) by reduction (to regenerate the NO_(x)occluding and reducing catalyst).

However, it has been known that when the rich-spike operation iseffected for the engine 1, the unpurified NO_(x) is released from theNO_(x) occluding and reducing catalyst immediately after the leanair-fuel ratio is changed over to the rich air-fuel ratio. This isattributed to the components HC and CO becoming in short supply in theexhaust gas when the engine is changed from the lean air-fuel ratiooperation over to the rich air-fuel ratio operation. That is, theair-fuel ratio in the exhaust gas continuously changes when it ischanged from the lean side over to the rich side. At this moment, thoughthe air-fuel ratio may be rich, the degree of richness is not very high,and a region where the amount of the HC and CO in the exhaust gas isrelatively small must be passed through. In this region where the HC andCO components are in short supply in the exhaust gas, therefore, it isnot considered that the NO_(x) released from the NO_(x) occluding andreducing catalyst is all reduced.

In this embodiment, therefore, when the NO_(x) is to be released fromthe NO_(x) occluding and reducing catalyst, the secondary fuel isinjected during the expansion or the exhaust stroke after the main fuelinjection in order to quickly change the air-fuel ratio of the exhaustgas to a considerably rich air-fuel ratio, so that the unpurified NO_(x)will not be released from the NO_(x) occluding and reducing catalyst.After the main fuel injected into the cylinder has burned, the fuel isinjected in the expansion or the exhaust stroke and remains unburned andcomes in contact with the burned gas of a high temperature, and isvaporized to form hydrocarbons of low molecular weights. Besides, thefuel supplied by the secondary fuel injection does not contribute to thecombustion in the cylinder. Therefore, even when the fuel is supplied ina relatively large amount by the secondary fuel injection, the outputtorque of the engine does not increase. When the secondary fuel isinjected when the NO_(x) is to be released from the NO_(x) occluding andreducing catalyst, therefore, the air-fuel ratio of the exhaust gas canbe quickly changed down to a low value without causing a change in theoutput torque of the engine. It is therefore possible to supply theexhaust gas having a high rich degree to the NO_(x) occluding andreducing catalyst without passing through a region of intermediateair-fuel ratios. This prevents the release of unpurified NO_(x) from theNO_(x) occluding and reducing catalyst in the early period of the NO_(x)releasing action. The NO_(x) may be released from the NO_(x) occludingand reducing catalyst relying on the secondary fuel injection only or byeffecting the secondary fuel injection at the initial period only of therich-spike operation at the time of conducting the normal rich-spikeoperation by increasing the amount of main fuel injection thereby toquickly change the air-fuel ratio of the exhaust gas to a rich air-fuelratio.

However, when the fuel supplied by the secondary fuel injection partlyremains in the cylinder, there may occur a change in the output torqueof the engine. As described above, the ECU 30 calculates the requiredamount of fuel based on the engine load conditions (degree ofaccelerator opening, rotational speed) and supplies the fuel into thecylinder by the main fuel injection. When the fuel due to the secondaryfuel injection remains in the cylinders, therefore, this remaining fuelburns in the cylinders in addition to the fuel supplied by the main fuelinjection of the next cycle; i.e., the output torque of the engineincreases due to the combustion of the fuel of an amount larger than therequired amount, and the torque changes.

According to the present invention, this problem is solved by thebelow-mentioned two methods.

(A) The fuel supplied by the secondary fuel injection is all dischargedout of the cylinder during the exhaust stroke (while the exhaust valveis opening), so that no fuel is left.

(B) When the fuel remains, the amount of injecting the main fuel in thenext time is corrected (decreased) by the remaining amount of fuel, sothat the amount of fuel that contributes to the combustion comes intoagreement with a target amount of main fuel injection.

Described below are the embodiments of when these methods are employed.

(1) First Embodiment.

In this embodiment, the fuel supplied by the secondary fuel injection isall discharged out of the cylinder during the exhaust stroke, in orderto prevent a change in the output torque of the engine caused by thesecondary fuel injection.

In FIG. 2, reference numeral 10 denotes a cylinder combustion chamber,11 denotes a piston, 13 denotes an intake port, 13 a denotes an intakevalve, 15 denotes an exhaust port, and 15 a denotes an exhaust valve.Further, reference numeral 111 denotes a direct cylinder fuel injectionvalve, and 17 denotes a spark plug provided at a central portion in thecylinder head. In this embodiment, a recessed piston cavity 11 a isformed in the top surface of the piston 11. The cavity 11 a works toconcentrate the fuel injected from the fuel injection valve 111 in thelatter half of the compression stroke during the operation at a leanair-fuel ratio, to the vicinity of the spark plug 17 to form a charge ofa mixture of a combustible air-fuel ratio near the plug 17. That is, inthe main fuel injection for the above-mentioned {circle around (1)} leanair-fuel ratio stratified charge combustion (fuel is injected in thecompression stroke) and {circle around (2)} lean air-fuel ratio uniformmixture/stratified charge combustion (fuel is injected in the suctionstroke and in the compression stroke), the fuel having a relativelystrong piercing force (having a high injection pressure) is injectedfrom the direct cylinder fuel injection valve 111 toward the pistoncavity 11 a at a moment when the piston arrives at a sufficientlyelevated position in the latter half of the compression stroke.

At this moment, the injected fuel arrives at the surface of the pistoncavity 11 a and flows along the curved surface of the cavity 11 a. Thecavity 11 a has a side surface 11 b having a relatively small radius ofcurvature on the side remote from the fuel injection valve 111, so thatthe fuel flowing along the surface of the cavity 11 a is deflectedtoward the vicinity of the spark plug 17. Accordingly, the fuel injectedfrom the fuel injection valve 111 forms a stratified charge near thespark plug 17.

In this embodiment, all the fuel supplied by the secondary fuelinjection is discharged through the exhaust port 15 by utilizing thepiston cavity 11 a. That is, in this embodiment, the timing forinjecting the fuel is set at a point delayed by 360 degrees in terms ofthe crank angle behind the timing for injecting the main fuel. When thesecondary fuel is injected, therefore, the piston 11 is assuming thesame position as when the main fuel is injected (hereinafter referred toas “fuel injection in the compression stroke”) for forming thestratified charge of the mixture. Accordingly, the fuel supplied by thesecondary fuel injection is deflected along the curved surface 11 b inthe same manner as the main fuel injection and flows toward the vicinityof the spark plug 17 (i.e., toward the exhaust port 13). Here, theexhaust valve 15 a is opening in the latter half of the exhaust stroke,and the deflected secondary fuel does not form a stratified charge aboutthe spark plug 17 as designated at F in FIG. 2, and is all dischargedout of the cylinder through the exhaust port 15. Therefore, thesecondary fuel does not remain in the cylinder. Here, the injected fuelcomes in contact with the surface of the piston cavity 11 a. However,the piston which is in operation is heated at a high temperature, andthe fuel that comes in contact with the surface of the cavity 11 a isreadily vaporized, and does not adhere or remain on the surface of thecavity 11 a.

In order to discharge all fuel supplied by the secondary fuel injectionout of the cylinder, the fuel must have been entirely discharged throughthe exhaust port 15 before the intake valve 13 a starts opening in theexhaust stroke. If the fuel remains in the cylinder during a period inwhich both the exhaust valve 15 a and the intake valve 13 a are opening(valve overlapping period), the fuel partly flows in a reverse directiontoward the intake port and flows again into the cylinder in the nextsuction stroke; i.e., the secondary fuel may partly remain in thecylinder. The engine 1 of this embodiment is equipped with a variablevalve timing device 200, and the valve timing varies based on the engineload conditions. According to this embodiment, therefore, the intakevalve opening timing is read when the secondary fuel injection isexecuted, and the amount of the secondary fuel injection is changedrelying on the intake valve opening timing, so that the fuel supplied bythe secondary fuel injection will not reversely flow toward the intakeport and will not remain in the cylinder.

FIG. 3 is a flow chart illustrating the operation for controlling thefuel injection according to the embodiment. This operation is conductedby a routine executed by the ECU 30 at every predetermined crankshaftrotation angle.

When the operation starts in FIG. 3, it is judged at a step 301 whetherthe secondary fuel injection is requested. In this embodiment, theamount of NO_(x) absorbed by the NO_(x) occluding and reducing catalyst7 is estimated based on the engine operating conditions by using aroutine (not shown) that is separately executed. When the absorbedamount of NO_(x) has reached a predetermined value, the secondary fuelinjection (rich spike) is requested. Instead of estimating the absorbedamount of NO_(x), it may be so presumed that the amount of NO_(x)absorbed by the NO_(x) occluding and reducing catalyst has reached apredetermined value when a predetermined period of time has lapsed fromthe last execution of the NO_(x) releasing operation or when theintegrated value of the number of revolutions of the engine has reacheda predetermined value from the last execution of the NO_(x) releasingoperation of the previous time, and the secondary fuel injection may berequested.

When the secondary fuel injection is not requested at the step 301, theoperation immediately ends without executing the steps 303 through 321,and the secondary fuel is not injected. When the secondary fuelinjection is requested at the step 301, on the other hand, the step 303is executed to calculate a target value qinjex of the amount of thesecondary fuel injection. At the step 303, the secondary fuel injectionamount qinjex necessary for obtaining a desired air-fuel ratio iscalculated from the amount of the air Q taken in by the cylinder per arevolution of the engine 1 and from the amount of main fuel injection.In this embodiment, the rotational speed N of the engine, load (degreeof accelerator opening) ACCP, and the amount of the air Q taken in bythe cylinder per a revolution of the engine are measured in advanceunder various operating conditions to find a relationship among Q, N andACCP. Based on these measured results, the values of the amount of theair Q taken in by the cylinder were stored in the ROM of the ECU 30 inthe form of a numerical value table using N and ACCP as parameters.Likewise, the amounts of main fuel injection were stored in the ROM ofthe ECU 30 as a numerical value table of N and ACCP. At the step 303,therefore, the amount of the air Q taken in and the amount of main fuelinjection are calculated from these numerical tables using the presentload conditions (N, ACCP), thereby to calculate the secondary fuelinjection amount qinjex necessary for bringing the air-fuel ratio of theexhaust gas to a target value.

The secondary fuel injection amount qinjex (milliliters) that iscalculated is then converted at a step 305 into a fuel injection time(fuel injection valve opening time) tauex (milliseconds) by using a fuelpressure in the common rail 110 and a characteristic value of the directcylinder fuel injection valve.

At a step 307, the intake valve opening timing (crank angle) IOcurrently set by the variable valve timing device 200 is read and at astep 309, a maximum secondary fuel injection time (guard value) tauexmaxthat can now be permitted is calculated.

In this embodiment as described above, the fuel supplied by thesecondary fuel injection from the fuel injection valve must all bedischarged out of the cylinder before the intake valve opens. In thisembodiment, further, the timing for injecting the secondary fuel hasbeen fixed (delayed by 360 degrees behind the injection timing in thecompression stroke). In order to discharge all the injected fuel out ofthe cylinder before the intake valve opens, therefore, a maximum amountof fuel injection must be limited. At the step 309, the time t₁(milliseconds) is calculated from the start of fuel injection to theopening of intake valve by using the present rotational speed N of theengine and a difference between the intake valve opening crank angle IOread at the step 307 and the secondary fuel injection start crank angleainjc+360 (ainjc is a timing for injecting the main fuel in thecompression stroke). In order for the fuel injected in the last stage ofthe secondary fuel injection to be discharged through the exhaust port,further, a time t₂ is required for the fuel to move from the fuelinjection valve to the exhaust port. Here, t₂ is determined by thepressure in the common rail 110. In this embodiment, therefore, the fuelmay remain in the cylinder unless the injection of fuel from the fuelinjecton valve is finished within a time (t₁−t₂) after the start of thefuel injection. At the step 309, therefore, the times t₁ and t₂ arecalculated by using the intake valve opening crank angle IO, rotationalspeed N of the engine and fuel pressure in the common rail, and amaximum fuel injection time tauexmax is calculated as tauexmax=t₁−t₂.

Then, at steps 311 to 317, the target secondary fuel injection timetauex set at the step 305 is limited by a maximum value tauexmax and aminimum value taumin thereby to set a value tauex to lie within a rangetaumin≦tauex≦tauexmax. The minimum value tauemin is a minimumcontrollable valve opening time of the fuel injection valve 111 and is acharacteristic value of the fuel injection valve 111.

At a step 319, the secondary fuel injection start timing ainjex is setto be ainjex=ainjc+360 and at a step 321, ainjex and tauex are set to afuel injection circuit (not shown). Then, the secondary fuel injectionstarts at a crank angle ainjex and continues for a time tauex.

According to this embodiment as described above, the fuel supplied bythe secondary fuel injection is deflected toward the exhaust port 13 byutilizing the piston cavity 11 a, and the amount of fuel injection iscontrolled based on the engine load conditions and the valve timing, sothat the fuel supplied by the secondary fuel injection is all dischargedout of the cylinder during the exhaust stroke.

In the embodiment of FIG. 2, the flow of fuel supplied by the secondaryfuel injection is deflected toward the exhaust port by the cavity 11 aformed in the top surface of the piston. However, the flow of fuel maybe deflected toward the exhaust port by using other means.

For example, the fuel injection valve may be constituted as an airassist valve that injects the compressed air together with the fuel, andthe compressed air may be injected toward the exhaust port only at thetime of the secondary fuel injection, so that the injected fuel isdeflected toward the exhaust port while being assisted by the air.

When the fuel injection valve is so constructed as to change thedirection of injection depending on the main fuel injection and thesecondary fuel injection, the direction of fuel injection may be changedtoward the exhaust port or toward the deflecting flow that is headed tothe exhaust port at the time when the secondary fuel is injected.

In the embodiment of FIG. 2, the main fuel injection and the secondaryfuel injection are effected using the same fuel injection valve. It is,however, also allowable to provide an auxiliary fuel injection valveexclusively for the secondary fuel injection separately from the fuelinjection valve for main fuel injection, and the direction of injectionof the auxiliary fuel injection valve may be set being pointed to theexhaust port.

Instead of the piston cavity, there may be provided a deflecting platethat protrudes into the cylinder at the time of the secondary fuelinjection only, and the flow of fuel supplied by the secondary fuelinjection may be brought into collision with the deflecting plate so asto be headed toward the exhaust port.

(2) Second Embodiment.

Next, a second embodiment of the invention will be described. In thisembodiment also, the fuel supplied by the secondary fuel injection isall discharged out of the cylinder during the exhaust stroke to preventa change in the output torque of the engine caused by the secondary fuelinjection, as in the above-mentioned first embodiment.

FIG. 4 is a sectional view of the cylinder of the engine 1 and issimilar to FIG. 2. In FIG. 4, reference numerals same as those of FIG. 2denote the elements same as those of FIG. 2.

In this embodiment, the secondary fuel injection is effected at an earlytiming in the exhaust stroke in which the piston is located at aposition close to the bottom dead center, and the pressure for thesecondary fuel injection (common rail pressure) is set to be lower thanthe pressure for the main fuel injection. At an early timing in theexhaust stroke, the burned gas is produced a high pressure in thecylinder, and a relatively strong exhaust flow is produced in thecylinder and moves toward the exhaust port as indicated by arrows inFIG. 4. If the secondary fuel is injected with a relatively low pressureat this early timing, then, the injected fuel does not arrive at thecylinder wall or the piston piercing through the exhaust flow in thecylinder but rides on the exhaust flow near the center of the cylinderand is conveyed to the exhaust port as designated at F in FIG. 4.Accordingly, the injected fuel does not adhere to the cylinder wall,piston or cylinder head; i.e., the fuel supplied by the secondary fuelinjection is all discharged out of the cylinder during the exhauststroke and does not remain in the cylinder.

In the above-mentioned embodiment, the amount of secondary fuelinjection is controlled based on the engine operating conditions (loadconditions) and the valve timings in order to discharge all fuelsupplied by the secondary fuel injection out of the cylinder before theintake valve is opened. In this embodiment, however, the timing of thesecondary fuel injection plays an important role. That is, as the fuelinjection timing is delayed, the fuel that is injected may partly flowreversely toward the intake port. When the fuel injection timing is tooearly, on the other hand, the injected fuel is diffused in the cylinderand may not all be discharged out of the cylinder riding on the exhaustflow. Next, described below is the operation for controlling the fuelinjection timing according to this embodiment.

FIG. 5 is a flow chart illustrating the operation for controlling thefuel injection according to the embodiment. This operation is conductedby a routine executed by the ECU 30 at every predetermined crankshaftrotation angle.

When the operation starts in FIG. 5, it is judged at a step 501 whetherthe secondary fuel injection is requested. At a step 503, a target valueqinjex of the secondary fuel injection amount is calculated and at astep 505, the injection time tauex is calculated from the targetinjection amount qinjex. The target injection amount qinjex at the step503 and the injection time tauex at the step 505 are calculated by thesame methods as those of the steps 303 and 305 in FIG. 3. In thisembodiment, however, the pressure in the common rail 110 for thesecondary fuel injection is controlled to be smaller than the pressurefor the main fuel injection. Even when the target injection amountqinjex is the same as the case of FIG. 3, therefore, the injection timetauex becomes longer than that of the case of FIG. 3.

After qinjex and tauex are calculated in this embodiment, the presentintake valve opening timing (crank angle) IO and the exhaust valveopening timing (crank angle) EO are read at a step 507, and, at a step509, a flying time t₂ of the fuel from when it is injected from the fuelinjection valve 111 until it is discharged through the exhaust port iscalculated as t₂=α+β. Here, α is the time required by the fuel from whenit is injected through the fuel injection valve 1112 until when itarrives at the central portion of the cylinder, and varies in proportionto the pressure in the common rail 110 (fuel injection pressure), and βis the time required by the fuel from when it has arrived at the centralportion until when it is discharged through the exhaust port riding onthe exhaust flow, and varies in proportion to the rotational speed N ofthe engine.

After the flying time t₂ is calculated at the step 509, a maximum value(guard value) tauexmax of the secondary fuel injection time iscalculated at a step 511 as tauexmax=t₃−t₂Here, t₃ is the time from whenthe exhaust valve is opened until when the intake valve is opened, andis calculated by using the valve opening timings IO, EO of the intakeand exhaust valves read at the step 507 and by using the rotationalspeed N of the engine. That is, tauexmax is an injection timecorresponding to a maximum fuel injection amount that can be alldischarged out of the cylinder without reversely flowing into the intakeport when the secondary fuel is injected simultaneously with the openingof the discharge valve.

After tauexmax is calculated at the step 511, the target injection timetauex calculated at the step 505 is limited at the steps 513 to 517 bythe maximum value tauexmax and the minimum value taumin. At a step 521,the secondary fuel injection start timing ainjex is calculated based onthe intake valve opening timing IO, rotational speed N of the engine andthe value of tauex after being limited. That is, the secondary fuelinjection starts before the intake valve starts opening by the amount oftime (tauex+t₂), i.e., starts at a timing at which the fuel injected atthe end of the secondary fuel injection can be discharged through theexhaust port just before the intake valve opens. That is, in thisembodiment, the secondary fuel injection timing is advanced with anincrease in the amount of the secondary fuel injection. Therefore, thefuel supplied by the secondary fuel injection all arrives at the exhaustport before the intake valve opens, and is also prevented from diffusingin the cylinder, caused by the secondary fuel injection timing being tooearly.

In the above-mentioned second embodiment, the direction of fuelinjection by the direct cylinder fuel injection valve 111 is the same asthat of the embodiment of FIG. 2. AS shown in FIG. 6, however, thedirect cylinder fuel injection valve may be provided being directeddownward at the central portion of the cylinder head. At the time of thesecondary fuel injection, the fuel is injected with a low pressuretoward the central portion of the cylinder, which is more effective forpreventing the fuel from remaining in the cylinder.

(3) Third Embodiment.

Described below is a third embodiment of the present invention. In thisembodiment, the amount of main fuel injection is corrected based on aprerequisite that the fuel supplied by the secondary fuel injectionpartly remains in the cylinder. That is, in this embodiment, the amountof fuel remaining in the cylinder due to the secondary fuel injection iscalculated from the engine operating conditions, and the amount of mainfuel injection in the next time is decreased by the remaining amount offuel. Thus, the amount of fuel that contributes to the combustion in thecylinder due to the main fuel injection comes into correct agreementwith the target amount of main fuel injection. Even when the fuelremains due to the secondary fuel injection, therefore, the change inthe output torque of the engine does not occur.

First, described below is the method of calculating the remaining amountof fuel due to the secondary fuel injection according to thisembodiment.

FIG. 7 is a graph illustrating a relationship between the output torqueof the engine (ordinate) and the amount of main fuel injection(abscissa) of when the engine is operated at a predetermined rotationalspeed. In FIG. 7, a curve I represents a relationship between the outputtorque and the amount of main fuel injection when the main fuel only isinjected without injecting the secondary fuel, and a curve II representsa relationship between the output torque and the amount of main fuelinjection when the secondary fuel is injected in addition to the mainfuel injection. As described earlier, the amount of the secondary fuelinjection has been determined in a manner that the air-fuel ratio of theexhaust gas becomes a target air-fuel ratio in their respective cases,and is determined based on the load on the engine (main fuel injection)and the rotational speed. In this case, further, the timing for thesecondary fuel injection may be set by any one of the above-mentionedmethods or may be fixed to a predetermined suitable crank angle.

As described earlier, when the fuel due to the secondary fuel injectiondoes not remain in the cylinder, the output torque of the engine remainsthe same irrespective of whether the secondary fuel is injected. Whenthe fuel remains in the cylinder due to the secondary fuel injection,however, the output torque of the engine increases by an amountcorresponding to the amount of the remaining fuel (curve II in FIG. 7).

It is now presumed that the output torque has increased with a givenamount of main fuel injection due to the injection of the secondary fuelas represented by a in FIG. 7. It is further presumed that the amount ofincrease in the output torque is equal to an increase in the amount ofmain fuel injection by an amount represented by b in FIG. 7. In thiscase, the increase in the output torque is caused by the combustion ofthe fuel remaining in the cylinder due to the secondary fuel injection.Therefore, the amount of the remaining fuel will become equal to theamount of main fuel injection necessary for increasing the outputtorque, i.e., will become equal to the amount of fuel denoted by b inFIG. 7. In this embodiment, therefore, when an increase in the outputtorque due to the secondary fuel injection is as denoted by a in FIG. 7,the remaining amount of fuel is estimated presuming that the amountrepresented by b in FIG. 7 is equal to the amount of fuel remaining inthe cylinder.

According to this embodiment, curves corresponding to those of FIG. 7are prepared in advance through experiment for every combination of therotational speeds of the engine and the fuel injection modes {circlearound (1)} to {circle around (5)} described below, and the amount ofthe remaining fuel (b in FIG. 7) is calculated in each of the main fuelinjection amounts. The amounts of the remaining fuel b are prepared inthe form of a numerical value table using the engine rotational speed Nand the main fuel injection amount (qinj=qinjei+qinjec) as parametersfor each of the fuel injection modes, and are stored in the ROM of theECU 30. When the engine is in operation, the ECU 30 calculates theamount of fuel remaining in the cylinder when the secondary fuel isinjected based on the engine rotational speed N and the main fuelinjection amount qinj. Here, qinjei is the amount of the first main fuelinjection during the suction stroke, qinjec is the amount of the secondmain fuel injection in the compression stroke, and qinj is the sum ofthe two.

In this embodiment, the engine 1 is also operated in the below-mentionedfive kinds of fuel injection modes.

{circle around (1)} A lean air-fuel ratio stratified charge combustion(fuel is injected in the compression stroke).

{circle around (2)} A lean air-fuel ratio uniform mixture/stratifiedcharge combustion (fuel is injected in the suction stroke and in thecompression stroke).

{circle around (3)} A lean air-fuel ratio uniform mixture/stratifiedcharge combustion (fuel is injected in the suction stroke).

{circle around (4)} A stoichiometric air-fuel ratio uniform mixturecombustion (fuel is injected in the suction stroke).

{circle around (5)} A rich air-fuel ratio uniform mixture combustion(fuel is injected in the suction stroke).

In this embodiment, when the mode {circle around (2)} (fuel is injectedin the suction stroke and in the compression stroke) is selected, thecorrection is effected by decreasing the amount of fuel injection in thesuction stroke by the remaining amount of fuel. The remaining fueldiffuses in the cylinder to become part of the uniform mixture and,hence, directly affects the air-fuel ratio of the uniform mixture formedin the cylinder. In order to prevent this, the amount of the fuelinjection in the suction stroke for forming the uniform mixture isdecreased by the remaining amount of fuel, so that the air-fuel ratio ofthe uniform mixture actually formed is maintained at a target value.

FIG. 8 is a flow chart illustrating the operation for controlling themain fuel injection according to the embodiment. This operation isexecuted at every predetermined crankshaft rotation angle of the engine.

In FIG. 8, when the operation starts, the engine load (degree ofaccelerator opening) ACCP and the rotational speed N are read at a step801. At steps 803, 811 and 823, it is determined which of the fuelinjection modes {circle around (1)} to {circle around (4)} shall beemployed based on the degree of accelerator opening ACCP and therotational speed N.

At the step 803, it is determined from ACCP and N whether one of thefuel injection mode of {circle around (3)} lean air-fuel ratio uniformmixture combustion (fuel is injected in the suction stroke), {circlearound (4)} stoichiometric air-fuel ratio uniform mixture combustion(fuel is injected in the suction stroke), or {circle around (5)} richair-fuel ratio uniform mixture combustion (fuel is injected in thesuction stroke) is employed. When the mode {circle around (3)} or{circle around (4)}, {circle around (5)} is employed, the suction strokefuel injection amount qinjei is calculated at the step 805 based on ACCPand N from the numerical value table that has been stored in advance inthe ROM of ECU 30. At a step 807, the compression stroke fuel injectionamount qinjec is set to 0.

Then, at a step 809, the sum qinj of qinjei and qinjec is calculated,and the amount b of fuel remaining in the cylinder when the secondaryfuel is injected is calculated from the relationship of FIG. 7 in thefuel injection mode {circle around (3)} or {circle around (4)} based onthe rotational speed N and the amount of main fuel injection qinj. At astep 817, it is judged whether the secondary fuel injection is nowrequested. When the secondary fuel injection is requested, the operationproceeds to a step 819 where the suction stroke fuel injection amountqinjei is decreased by the remaining amount b of fuel and at a step 821,the suction stroke fuel injection amount qinjei after corrected and thecompression stroke fuel injection amount qinjec (quinjec=0 in this case)are set to the fuel injection circuit before terminating the operation.When the secondary fuel is injected, therefore, the amount of main fuelis decreased by an amount equal to the remaining amount b of fuel. Evenwhen the secondary fuel is injected, therefore, the output torque of theengine is prevented from changing.

When none of the fuel injection mode {circle around (3)}, {circle around(4)} or {circle around (5)} is employed at the step 803, the routineproceeds to a step 811 where it is judged whether one of the fuelinjection mode {circle around (2)} (lean air-fuel ratio uniformmixture/stratified charge combustion (fuel is injected in the suctionstroke and in the compression stroke)) is employed. When the mode{circle around (2)} is employed, the suction stroke fuel injectionamount qinjei and the compression stroke fuel injection amount qinjecare calculated at a step 813 from the numerical value table stored inadvance in the ROM of ECU 30 based on ACCP and N. At a step 815, theremaining amount b of fuel due to the secondary fuel injection iscalculated from the relationship of FIG. 7 in the mode {circle around(2)}, and the operations of the step 817 and subsequent steps arecarried out. In this case, also the suction stroke fuel injection amountqinjei only is decreased by the remaining amount b of fuel, but thecompression stroke fuel injection amount qinjec is not corrected.

When none of the fuel injection modes {circle around (2)} to {circlearound (5)} is employed at the steps 803 and 801, then, the fuelinjection mode {circle around (1)} (lean air-fuel ratio stratifiedcharge combustion (fuel is injected in the compression stroke)) isemployed. In this case, the compression stroke fuel injection amountqinjec is calculated based on ACC and N at a step 823, and the suctionstroke fuel injection amount qinjei is set to 0. In this case, also theremaining amount b of fuel due to the secondary fuel injection iscalculated at the step 827 from the relationship of FIG. 7 in the mode{circle around (1)}, and it is judged at a step 829 whether thesecondary fuel injection is requested. When the secondary fuel injectionis requested at the step 829, the compression stroke fuel injectionamount qinjec is decreased by the amount of the remaining fuel b at astep 831, and the operation of the step 821 is executed.

According to this embodiment as described above, the amount of main fuelinjection is corrected based upon the remaining amount of fuel due tothe secondary fuel injection, and the output torque of the engine is notchanged by the secondary fuel injection. In effecting the correction inthe mode {circle around (3)} (fuel is injected in the suction stroke andin the compression stroke), further, the suction stroke fuel injectionamount only is corrected, so that the air-fuel ratio of the uniformair-fuel ratio mixture is brought into agreement with the target value.

In this embodiment, the correction is effected for each cycle by using arelationship between the amount of main fuel injection and the amount offuel remaining in the cylinder, that is found in advance throughexperiment. It is, however, also allowable to detect a change in theoutput torque of the engine (change represented by a in FIG. 7) causedby the fuel remaining in the cylinder from a change in the rotationalspeed of the engine or from a change in the combustion pressure in thecylinder, and calculate the amount b for correcting the amount of mainfuel injection from the relationship of FIG. 7 based on the amount ofchange in the torque. In this case, the amount of main fuel injection iscorrected in a cycle next of the cycle in which the change in the torquewas detected.

The foregoing embodiments have dealt with the engine which changes overthe fuel injection mode based on the engine operating conditions.However, it needs not be pointed out that the present invention can alsobe applied to the engine in which the fuel injection mode is fixed tothe suction stroke fuel injection or the compression stroke fuelinjection or to both of them (injected twice), as a matter of course.

Further, the foregoing embodiments have dealt with the case where theNO_(x) occluding and reducing catalyst was disposed in the exhaustpassage. However, it needs not be pointed out that the invention is inno way limited thereto only but can be applied to any case where thesecondary fuel is injected. For example, when a selectively reducingcatalyst is disposed in the exhaust passage to reduce the NO_(x) byselectively reacting HC in the exhaust gas (or HC adsorbed by thecatalyst) with NO_(x) under the conditions of a lean air-fuel ratio, itis required to supply HC to the selectively reducing catalyst. Thepresent invention can be applied even to the case where the HC issupplied to the selectively reducing catalyst by the secondary fuelinjection.

(4) Fourth Embodiment.

Next, described below is a fourth embodiment of the present invention.

FIG. 9 schematically illustrates the constitution of this embodiment andis similar to FIG. 1. In FIG. 9, reference numerals same as those ofFIG. 1 denote the same elements.

The embodiment of FIG. 9 is different from the embodiment of FIG. 1 withrespect to the provision of an exhaust gas recirculation device forrecirculating part of the exhaust gas of the engine into the engineintake air system. Though not indicated in FIG. 9, the engine 1 in thisembodiment, too, is equipped with the common rail 110, throttle valve15, etc. like in the embodiment of FIG. 1.

In this embodiment as shown in FIG. 9, the upstream side of the SC 5 bof the exhaust passage 2 b of the cylinders #2 and #3 is connected tothe surge tank 10 a of the engine intake passage 10 through an EGRpassage 43. Further, an EGR valve 41 comprising a flow rate controlvalve is provided in the EGR passage 43 to control the flow rate of theexhaust gas recirculating from the exhaust passage 2 b to the intakepassage 10 through the EGR passage. The EGR valve 41 is equipped with anactuator 41 a of a suitable form such as step motor, negative-pressureactuator, etc. that operates in response to a control signal from theECU 30, and determines its opening degree based on the control signalfrom the ECU 30.

In this embodiment, too, the ECU 30 changes the fuel injection mode ofthe direct cylinder injection valves 111 to 114 in the same manner as inthe embodiment of FIG. 1, and operates the engine in any one of thebelow-mentioned five modes based on the engine operating conditions.

{circle around (1)} A lean air-fuel ratio stratified charge combustion(fuel is injected in the compression stroke).

{circle around (2)} A lean air-fuel ratio uniform mixture/stratifiedcharge combustion (fuel is injected in the suction stroke and in thecompression stroke).

{circle around (3)} A lean air-fuel ratio uniform mixture/stratifiedcharge combustion (fuel is injected in the suction stroke).

{circle around (4)} A stoichiometric air-fuel ratio uniform mixturecombustion (fuel is injected in the suction stroke).

{circle around (5)} A rich air-fuel ratio uniform mixture combustion(fuel is injected in the suction stroke).

In this embodiment, further, the ECU 30 controls the EGR valve 41 torecirculate part of the exhaust gas into the intake passage 10 from theexhaust passage 2 b based on the engine operating conditions. In otherwords, this embodiment executes the external EGR. In order to controlthe amount of EGR, the output port of the ECU 30 is connected to theactuator 41 a of the EGR valve through a drive circuit (not shown) tocontrol the opening degree of the EGR valve 41.

In this embodiment, too, the ECU 30, as required, executes the secondaryfuel injection from the direct cylinder fuel injection valves 111 to 114into the respective cylinders during the expansion stroke or the exhauststroke while the engine is in operation, in order to change the air-fuelratio of the exhaust gas from the engine independently of the engineoperating air-fuel ratio.

In this embodiment, as the amount of NO_(x) absorbed by the NO_(x)occluding and reducing catalyst 7 increases during the operation at alean air-fuel ratio, the ineffective fuel that does not burn in thecylinder is supplied to the engine so that the exhaust gas from theengine becomes a rich air-fuel ratio, thereby to release NO_(x) from theNO_(x) occluding and reducing catalyst and to purify the NO_(x) byreduction (to regenerate the NO_(x) occluding and reducing catalyst).

In this embodiment, the ECU 30 increases or decreases the value of anNO_(x) counter in order to estimate the amount of NO_(x) absorbed andheld by the NO_(x) occluding and reducing catalyst 7. The amount ofNO_(x) absorbed by the NO_(x) occluding and reducing catalyst 7 per aunit time varies in proportion to the amount of NO_(x) in the exhaustgas flowing into the NO_(x) occluding and reducing catalyst per a unittime, i.e., varies in proportion to the amount of NO_(x) generated bythe engine 1 per a unit time. On the other hand, the amount of NO_(x)generated by the engine per a unit time is determined by the amount offuel fed to the engine, air-fuel ratio, flow rate of the exhaust gas,etc. When the operation conditions of the engine are determined,therefore, it is possible to know the amount of NO_(x) absorbed by theNO_(x) occluding and reducing catalyst. According to this embodiment,the engine operating conditions (degree of accelerator opening, enginerotational speed, amount of the air taken in, intake air pressure,air-fuel ratio, amount of feeding fuel, etc.) are changed to measure theamount of NO_(x) generated by the engine per a unit time, and the amountof NO_(x) absorbed by the NO_(x) occluding and reducing catalyst 7 per aunit time is stored in the ROM of ECU 30 in the form of a numericalvalue table using, for example, load on the engine (amount of fuelinjection) and the engine rotational speed as parameters. The ECU 30calculates the amount of NO_(x) absorbed by the NO_(x) occluding andreducing catalyst per a unit time after every predetermined period oftime (after every unit time) by using the load on the engine (amount offuel injection) and the engine rotational speed, and increases the valueof the NO_(x) counter by the amount of NO_(x) absorbed. Therefore, thevalue of the NO_(x) counter always indicates the amount of NO_(x)absorbed by the NO_(x) occluding and reducing catalyst 7. When the valueof the NO_(x) counter exceeds a predetermined value while the engine isin operation at a lean air-fuel ratio, the ECU 30 supplies to the enginethe fuel that does not burn in the combustion chamber to change theair-fuel ratio of the exhaust gas of the engine over to a rich air-fuelratio. Therefore, the exhaust gas having a rich air-fuel ratio flowsinto the NO_(x) occluding and reducing catalyst irrespective of theengine operating air-fuel ratio. Therefore, the absorbed NO_(x) isreleased from the NO_(x) occluding and reducing catalyst and is purifiedby reduction. The time for maintaining the exhaust gas air-fuel ratiorich by supplying the ineffective fuel is experimentally determinedbased upon the kind and volume of the NO_(x) occluding and reducingcatalyst. The value of the NO_(x) counter is reset to 0 after the NO_(x)is released from the NO_(x) occluding and reducing catalyst and ispurified by reduction upon supplying the ineffective fuel. Uponsupplying the ineffective fuel based on the amount of NO_(x) absorbed bythe NO_(x) occluding and reducing catalyst 7 as described above, theNO_(x) occluding and reducing catalyst 7 is properly regenerated and isnot saturated with the absorbed NO_(x).

As described above, the ineffective fuel is supplied by the two methods,i.e., a method which injects the secondary fuel from the direct cylinderfuel injection valve into the cylinder in the expansion stroke or theexhaust stroke of the cylinder, and a method which injects the fuel intothe exhaust port. In this embodiment having the direct cylinder fuelinjection valves 111 to 114, the ineffective fuel is supplied to theengine by the secondary fuel injection. The present invention, however,can be similarly applied even to the engine equipped with exhaust portfuel injection valves and in which the ineffective fuel is supplied bythe exhaust port fuel injection.

When the ineffective fuel is supplied to the engine by the secondaryfuel injection (or exhaust port fuel injection) as described above, theexhaust gas of the engine contains relatively large amounts of unburnedfuel supplied as the ineffective fuel. When the ineffective fuel issupplied while the EGR is being executed, therefore, the exhaust gascontaining relatively large amounts of unburned fuel is recirculatedinto the intake passage 10 through the EGR passage 43, and the unburnedfuel is supplied into the combustion chamber in the cylinder and isburned in the combustion chamber. As described above, however, theamount of fuel injection into the engine (hereinafter referred to as theamount of main fuel injection to make a distinction over the ineffectivefuel) is controlled to an optimum value by the ECU 30 based on theengine operating conditions. When the unburned fuel supplied into thecombustion chamber together with the recirculated exhaust gas burns,therefore, the amount of fuel supplied into the engine becomesexcessive, and the air-fuel ratio becomes too rich causing thecombustion to become unstable, or the rich combustion air-fuel ratiocauses the engine to produce an increased output, resulting in a changein the engine output torque.

When the secondary fuel is injected according to this embodiment,therefore, the EGR is limited to solve the above-mentioned problem. Thatis, the EGR is limited (e.g., interrupted), whereby no exhaust gascontaining the unburned fuel recirculates into the combustion chamber ofthe engine. Therefore, the combustion air-fuel ratio in the combustionchamber varies in proportion to the amount of main fuel injection, andthe air-fuel ratio is prevented from becoming more rich than an optimumvalue.

FIG. 10 is a flow chart illustrating the operation of the secondary fuelinjection according to the embodiment. This operation is conducted by aroutine executed by the ECU 30 at every predetermined interval (e.g., atevery predetermined crankshaft rotation angle).

When the operation starts in FIG. 10, it is judged at a step 1001whether the secondary fuel injection is now requested. In thisembodiment, the ECU 30 requests the secondary fuel injection for apredetermined period of time only when the value of the NO_(x) counterCNOX of the NO_(x) occluding and reducing catalyst 7 becomes larger thana predetermined value.

When the secondary fuel injection is requested at the step 1001, thedegree of accelerator opening ACCP, the engine rotational speed NE andthe engine intake air pressure PM are read at a step 1003 and where thepresent operation mode ({circle around (1)} to {circle around (5)}) isjudged from ACCP and NE. Further, the present main fuel injection amountqINJ of the engine is calculated from the numerical value table preparedfor each of the operation modes based on ACCP and NE (modes {circlearound (1)} to {circle around (3)}) or based on PM and NE (modes {circlearound (4)} and {circle around (5)}).

At a step 1007, the EGR is interrupted. In this embodiment, the EGR isinterrupted by fully closing the EGR valve 41. At a step 1009, it isjudged whether the EGR is interrupted by the above operation (i.e.,whether the EGR valve 41 is fully closed). Steps 1011 and 1013 are notexecuted until the EGR is interrupted.

When the EGR is interrupted at the step 1009, the secondary fuelinjection amount q_(EX) is calculated at the step 1011. The secondaryfuel injection amount q_(EX) is calculated based on the main fuelinjection amount q_(INJ) and the engine operating air-fuel ratio so thatthe air-fuel ratio of the exhaust gas flowing into the NO_(x) occludingand reducing catalyst 7 becomes a predetermined rich air-fuel ratio. Atthe step 1013, the secondary fuel is injected into all cylinders in theexpansion stroke or in the exhaust stroke. When the secondary fuelinjection is not requested at the step 1001, the operation immediatelyterminates without injecting the secondary fuel. In this case, the EGRthat is being effected is allowed to continue.

In this embodiment, the EGR is interrupted when the secondary fuel isinjected, to prevent the unburned fuel from recirculating into theengine combustion chamber and to prevent the combustion from losingstability and the output torque from changing. In this embodiment, theEGR is completely interrupted when the secondary fuel is injected. Inthe actual operation, however, problems do not occur even if theunburned fuel is recirculated into the combustion chamber to some extentunless the combustion becomes unstable and the output torque changes.Therefore, a maximum EGR amount may be found in advance throughexperiment that does not cause problem even if the EGR is executed withthe secondary fuel being injected, and the EGR amount may be decreasedto a value not larger than the above maximum value when the secondaryfuel is injected.

(5) Fifth Embodiment.

The fourth embodiment has dealt with the case of the external EGRsystem. However, the same control operation can be applied to the caseof the internal EGR system, too.

FIG. 11 is a diagram schematically illustrating the constitution of afifth embodiment of the present invention using the internal EGR system.In FIG. 11, reference numerals the same as those of FIGS. 1 and 9 denoteelements similar to those of FIGS. 1 and 9.

The embodiment of FIG. 11 is provided with neither the EGR passage 43nor the EGR valve 41 of FIG. 9. Instead, the internal EGR is controlledby using a variable valve timing device 200 for varying the valvetimings of the engine 1. In this embodiment, any known variable valvetiming device 200 can be used provided it is capable of varying thevalve timings of the engine 1 based on control signals from the ECU 30.For example, there can be used any one for varying the open-closetimings of the intake valves and/or the exhaust valves, or for varyingthe open-close timings as well as the valve lift. The valve timings maybe varied either continuously or stepwisely.

FIG. 12 is a diagram illustrating the valve timings of the engine 1.FIG. 12 schematically illustrates general open-close timings of anintake valve and an exhaust valve, and where the open-close timings ofthe intake valve are changed by an equal amount. In FIG. 12, TDC is thetop dead center of the piston, BDC is the bottom dead center, IO and ICare the valve-opening timing and the valve-closing timing of the intakevalve, and EO and EC are the valve-opening timing and the valve-closingtiming of the exhaust valve. As shown in FIG. 12, the intake valve isopened before the top dead center (TDC) in the exhaust stroke and isclosed after the bottom dead center (BDC) in the suction stroke. Theexhaust valve is opened before the bottom dead center (BDC) in theexplosion stroke and is closed after the top dead center (TDC) in theexhaust stroke. In the exhaust stroke as shown in FIG. 12, the valvetiming has been so set that the intake valve is opened (IO) before theexhaust valve is closed (EC) and, hence, there exists a period in whichboth the intake valve and the exhaust valve are opened (OL in FIG. 12).In this embodiment, the length (angle) of the period OL is called valveoverlapping amount. In this embodiment, as will be described later, theintake valve timing (valve-opening timing) can be adjusted from a timingrepresented by IO₀ (most delayed timing) to a timing represented by IO₁(most advanced timing) shown in FIG. 12. In this embodiment, further,the crankshaft rotation angle from the most delayed valve timingposition (IO₀) to the present position (IO) is defined to be a valvetiming value VT. In this embodiment as will be understood from FIG. 12,the timing for closing the exhaust valve is fixed and, hence, the valvetiming value VT and the valve overlapping amount OL correspond to eachother in a 1:1 manner.

In general, the time in which the intake valve remains opened during theexhaust stroke increases with an increase in the valve overlappingamount OL of the intake and exhaust valves (with an increase in theintake valve timing VT). Therefore, the burned gas (exhaust gas) afterthe combustion in the cylinder flows reversely into the intake portthrough the intake valve that is opened, and is recirculated again intothe cylinder during the suction stroke. Therefore, the amount of exhaustgas (amount of EGR gas) recirculated into the engine combustion chamberincreases with an increase in the valve overlapping amount OL. Accordingto this embodiment, therefore, the ECU 30 adjusts the amount of theexhaust gas recirculating into the engine combustion chamber bycontrolling the intake valve timing VT (i.e., valve overlapping amountOL) instead of controlling the opening degree of the EGR valve 41 ofFIG. 9.

When the secondary fuel is injected into the cylinders during theexpansion stroke or the exhaust stroke, the ineffective fuel that isinjected partly flows reversely into the intake port together with theburned gas during the overlapping period and recirculates into thecombustion chamber during the suction stroke, causing the same problemas that of the external EGR. In this case, therefore, the ECU 30interrupts the EGR (sets the overlapping amount OL to 0) when thesecondary fuel is being injected, so that the combustion will not losestability and the output torque will not be changed by the recirculationof the unburned fuel.

The operation in this case is the same as that of the flow chart of FIG.10. However, the operation for interrupting the EGR at the step 1007 isexecuted by delaying the intake valve timing VT and by setting the valveoverlapping amount to be 0.

In this case, too, the EGR amount may be decreased to such a degree thatno problem occurs in practice when the secondary fuel is injected,instead of interrupting the EGR.

(6) Sixth Embodiment.

Described below is a sixth embodiment of the present invention.

In the fourth and fifth embodiments, the EGR is interrupted when thesecondary fuel is injected. In the embodiment described below, however,the injection of secondary fuel is inhibited when the EGR is executed.Since no secondary fuel is injected when the EGR is executed, nounburned fuel is contained in the exhaust gas recirculated into theengine combustion chamber. Similar to the above-mentioned embodiments,therefore, a problem caused by the recirculation of the unburned fuelinto the combustion chamber does not occur.

FIG. 13 is a flow chart illustrating the operation of the secondary fuelinjection when the secondary fuel injection is inhibited at the time ofexecuting the EGR in the engine of the external EGR system of FIG. 9.This operation is conducted by a routine executed by the ECU 30 atpredetermined interval (e.g., at predetermined crankshaft rotationangle).

In the engine 1 of FIG. 9, the EGR passage 43 is connected to theexhaust passage 2 b of the cylinders #2 and #3 of the engine. When theEGR is executed, therefore, the secondary fuel injection may beinhibited for the cylinders #2 and #3 to prevent the unburned fuel frombeing mixed into the recirculating exhaust gas. In this operation,therefore, the secondary fuel is not injected into the cylinders #2 and#3 when the EGR is executed, and the secondary fuel is injected into thecylinders #1 and #4 only.

When the operation starts in FIG. 13, it is judged at a step 1301whether the secondary fuel injection is now requested. When thesecondary fuel injection is requested, the degree of accelerator openingACCP, the engine rotational speed NE and the engine intake air pressurePM are read at a next step 1303 the present main fuel injection amountq_(INJ) is calculated. Operations at steps 1301 to 1305 are the same asoperations of the steps 1001 to 1005 of FIG. 10.

Next, it is judged at a step 1307 whether the EGR is now being effected.When the EGR is effected, the amount Q_(EX) of recirculating exhaust gasis calculated at a step 1309 based on the main fuel injection amountq_(INJ) and the rotational speed NE. Then, at a step 1311, the secondaryfuel injection amount q_(EX) is calculated based on the EGR amountQ_(EX) and the main fuel injection amount q_(INJ). In this embodiment,the secondary fuel injection amount q_(EX) is set in advance based onthe main fuel injection amount q_(INJ) and the amount Q_(EX) ofrecirculating exhaust gas, so that the air-fuel ratio of the exhaust gasflowing into the NO_(x) occluding and reducing catalyst 7 becomes apredetermined rich air-fuel ratio. When the EGR is effected, however, nosecondary fuel is injected to the cylinders #2 and #3. Therefore, theair-fuel ratio of the exhaust gas flowing into the NO_(x) occluding andreducing catalyst 7 must be maintained at a predetermined rich air-fuelratio by injecting the secondary fuel into the cylinders #1 and #4 only.In this embodiment, the secondary fuel injection amount q_(EX) is setpreviously based on the main fuel injection amount q_(INJ) and theamount Q_(EX) of circulating exhaust gas (based on combustion air-fuelratio in the cylinder determined by q_(EX) and Q_(EX)) for the casewhere the secondary fuel is injected into the cylinders #1 and #4 only,and is stored as a numerical value table using q_(INJ) and Q_(EX) asparameters in the ROM of the ECU 30. At a step 1311, therefore, thesecondary fuel injection amount q_(EX) is calculated from q_(INJ) andQ_(EX) based on the numerical value table. At a step 1313, the secondaryfuel is injected in the expansion stroke or in the exhaust stroke of thecylinders #1 and #4.

When the EGR is not being effected at the step 1307, the operationproceeds to a step 1315 where the secondary fuel injection amount q_(EX)is calculated. Here, since the EGR is not being effected, the secondaryfuel is injected into all cylinders, and the secondary fuel injectionamount q_(EX) is calculated based on the main fuel injection amountq_(INJ) and the engine operating air-fuel ratio like at the step 1011 inFIG. 10. At a step 1317, the secondary fuel is injected into allcylinders inclusive of the cylinders #2 and #3 in the expansion strokeor in the exhaust stroke. When the secondary fuel injection is notrequested at the step 1301, the secondary fuel is not injected, and theoperation is immediately terminated.

When the EGR is being executed as described above, the secondary fuelinjection is inhibited to completely prevent the unburned fuel frombeing mixed into the engine combustion chamber. In this embodiment, thesecondary fuel injection is completely inhibited when the EGR is beingeffected. However, in the actual operation, problems do not occur evenif the unburned fuel enters into the combustion chambers provided theamount of the unburned fuel is small enough so that it does not causethe combustion to lose stability or does not cause a change in thetorque. Therefore, a maximum secondary fuel injection amount may be setin advance through experiment in such a manner that it does not causethe combustion to lose stability or does not cause a change in thetorque when the EGR is being effected, and the secondary fuel injectionamount for the cylinders #2 and #3 may be decreased to a value smallerthan the maximum amount when the EGR is effected.

FIG. 14 is a flow chart for explaining the operation of the secondaryfuel injection when the secondary fuel injection is inhibited in theengine of the internal EGR system of FIG. 11 while the EGR is beingeffected. This operation is conducted by a routine executed by the ECU30 at every predetermined interval (e.g., at every predeterminedcrankshaft rotation angle).

When the operation starts in FIG. 14, it is judged at steps 1401 to 1405whether the secondary fuel injection is requested (step 1401), and ACCP,NE and PM are read (1403), and the main fuel injection amount q_(INJ) iscalculated (step 1405). Operations of the steps 1401 to 1405 are thesame as those of the steps 1301 to 1305 in FIG. 13. At a step 1407, itis judged whether the EGR is now being effected based on the enginevalve overlapping amount OL. When OL=0 (no overlapping), the internalEGR is not now being effected, and the operation proceeds to a step 1409where the secondary fuel injection amount q_(EX) is calculated. At astep 1411, the secondary fuel is injected into all cylinders in theexpansion stroke or in the exhaust stroke. Operations at the steps 1409and 1411 are the same as those of the steps 1315 and 1317 of FIG. 13. Inthis embodiment, on the other hand, when the EGR is being effected atthe step 1407, the secondary fuel is injected into none of thecylinders. That is, the secondary fuel injection is inhibited for allcylinders, and the unburned fuel is prevented from recirculating intothe combustion chambers.

In this embodiment, too, the secondary fuel may be injected but in sucha small amount that does not cause any problem instead of inhibiting thesecondary fuel from being injected into the cylinders while the EGR isbeing effected.

(7) Seventh Embodiment.

Next, described below is a further embodiment of the present invention.In the embodiments of FIGS. 13 and 14, the secondary fuel injection islimited while the EGR is being effected to prevent the unburned fuelfrom being recirculated into the combustion chambers. However, in theinternal EGR system of FIG. 11, in particular, limitation of thesecondary fuel injection makes it quite difficult to release NO_(x) fromthe NO_(x) occluding and reducing catalyst 7. When the secondary fuelinjection is limited whenever the EGR is effected, therefore, thefrequency of the operation for releasing NO_(x) from the NO_(x)occluding and reducing catalyst decreases, and the NO_(x) occluding andreducing catalyst tends to be saturated with the NO_(x) which it hasabsorbed. In an embodiment that will be described below, therefore, whenthe amount of recirculating exhaust gas while the EGR is being effectedis larger than a predetermined value, the secondary fuel injection isinhibited even when the secondary fuel injection is requested like inthe embodiment of FIG. 14. When the amount of recirculating exhaust gasis smaller than the predetermined value, however, contrary to theabove-mentioned operation, the secondary fuel is injected whileinterrupting the EGR. When the EGR is interrupted, the amount of NO_(x)emitted from the engine increases. However, the amount of NO_(x)emission does not greatly increase even when the EGR is interrupted whenthe amount of recirculating exhaust gas before the EGR is interrupted isrelatively small. When the amount of recirculating exhaust gas isrelatively small, therefore, it becomes advantageous, as a whole, toincrease the frequency for releasing NO_(x) by injecting the secondaryfuel while interrupting the EGR. In this embodiment, since the engineequipped with the internal EGR system is used as shown in FIG. 11, thesecondary fuel injection is inhibited when the amount of recirculatingexhaust gas by the EGR is larger than a predetermined value but,conversely, the secondary fuel is injected while interrupting the EGRwhen the amount of recirculating exhaust gas is smaller than thepredetermined value. This makes it possible to prevent the combustionfrom losing stability and the engine output torque from changing, due toby the recirculation of the unburned fuel into the combustion chambers,while maintaining a high frequency for executing the operation forreleasing NO_(x) from the NO_(x) occluding and reducing catalyst.

FIG. 15 is a flow chart illustrating the operation of the secondary fuelinjection according to the embodiment. This operation is conducted by aroutine executed by the ECU 30 at every predetermined interval (e.g., atevery predetermined crankshaft rotation angle).

In the operation of FIG. 15, it is judged at a step 1501 whether thesecondary fuel injection is now being requested. At steps 1503 and 1505,the main fuel injection amount q_(INJ) is calculated based on ACCP, NEand PM. The operations at the steps 1501 to 1505 are the same as thoseof the steps 1001 to 1005 of FIG. 10.

After the main fuel injection amount q_(INJ) is calculated at the step1505, it is judged at a step 1507, based on the valve overlapping amountOL, whether the amount of recirculating exhaust gas due to EGR is nowlarger than the predetermined amount. As described earlier, the amountof exhaust gas recirculating into the combustion chamber due to EGRincreases with an increase in the valve overlapping amount OL. In thisembodiment, therefore, when the valve overlapping amount OL is largerthan a predetermined value α, it is so judged that the amount of therecirculating exhaust gas due to EGR is now larger than thepredetermined value.

When the secondary fuel injection is not requested at the step 1501 andwhen the valve overlapping amount OL is larger than the predeterminedvalue α at the step 1507, the operation readily ends without executingthe operations of a step 1509 and of subsequent steps. That is, when theexhaust gas is recirculated in large amounts into the combustionchamber, the secondary fuel is not injected even when the secondary fuelinjection is requested.

On the other hand, when the valve overlapping amount OL is smaller thana at the step 1505, i.e., when the exhaust gas is recirculated in smallamounts by the EGR, the operation proceeds to the step 1509 to executethe interruption operation (i.e., to accomplish OL=0 by delaying theintake valve timing VT). After the interruption of EGR is confirmed at astep 1511, i.e., when OL=0 is established, the secondary fuel injectionamount q_(EX) is calculated at a step 1513, and, at a step 1515, thesecondary fuel is injected into all cylinders in the expansion stroke orin the exhaust stroke. Operations of the steps 1513 and 1515 are thesame as those of the steps 1011 and 1013 of FIG. 10.

When the valve overlapping amount OL is larger than the predeterminedvalue α, therefore, the secondary fuel injection is inhibited and theEGR continues. When the valve overlapping amount OL is smaller than thepredetermined value α, the EGR is interrupted and the secondary fuel isinjected. Therefore, the frequency for injecting the secondary fuelincreases (frequency for releasing NO_(x) from the NO_(x) occluding andreducing catalyst increases), and the NO_(x) occluding and reducingcatalyst is prevented from being saturated.

(8) Eighth Embodiment.

Described below is a still further embodiment of the present invention.

In the above-mentioned fourth to seventh embodiments, the secondary fuelinjection is limited when the EGR is being effected, or the EGR islimited when the secondary fuel is being injected. When the secondaryfuel injection is limited, however, the frequency for executing theoperation for releasing NO_(x) from the NO_(x) occluding and reducingcatalyst may decrease, and the efficiency for purifying the exhaust gasmay drop due to the saturation of the NO_(x) occluding and reducingcatalyst. When the EGR is limited, further, the amount of NO_(x) emittedfrom the engine increases. It is therefore desired to effect both theEGR and the secondary fuel injection simultaneously without limitation,if it is possible.

When the ineffective fuel is supplied while the EGR is being effected, aproblem occurs, since the unburned fuel is recirculated into thecombustion chamber together with the recirculating exhaust gas, in thatexcess fuel is supplied to the combustion chamber. Therefore, if thefuel is not supplied in excess amounts into the combustion chamber, theproblem does not occur even if the ineffective fuel is supplied whilethe EGR is being effected. In the embodiment described below, therefore,the amount of unburned fuel recirculated into the combustion chamber isestimated when the ineffective fuel is supplied while the EGR is beingeffected, and the amount of the main fuel injection is decreasedrelative to the target value by the amount of the unburned fuel. Evenwhen the unburned fuel recirculates, therefore, the sum of the amount offuel supplied to the engine combustion chamber is maintained to be thesame as the target value of the main fuel injection amount, and the fuelis not supplied in excess amounts. When the EGR is being effected,therefore, the secondary fuel is injected without being limited,preventing the combustion from losing stability and preventing a changein the output torque.

Prior to describing the operation for correcting the main fuel injectionamount according to this embodiment, mentioned below first is a methodof calculating the amount of unburned fuel recirculating into thecombustion chamber due to the supply of ineffective fuel at the timewhen the EGR is being effected in the embodiment. In this embodiment,the amount of recirculating of the unburned fuel by the supply ofineffective fuel at the time of effecting the EGR is calculated based onthe method which is nearly the same as the one described with referenceto FIG. 7.

FIG. 16 is a graph showing a relationship between the cylinder outputtorque (ordinate) of when the engine is operated at a predeterminedrotational speed and the amount of the main fuel injection (abscissa).In FIG. 16, a curve I represents a relationship between the outputtorque and the amount of main fuel injection of when the EGR is beingeffected, and a curve II represents a relationship between the outputtorque and the amount of main fuel injection of when the ineffectivefuel is supplied (secondary fuel in this case) while the EGR is beingeffected. As described earlier, the amount of the secondary fuel hasbeen set to be the one required for bringing the air-fuel ratio of theexhaust gas to the target air-fuel ratio for each of the cases, and isdetermined from the engine load (main fuel injection) and the rotationalspeed. Similarly, the amount of recirculating the exhaust gas by the EGRis determined from the engine load and the rotational speed.

If the unburned fuel does not recirculate by the secondary fuelinjection into the combustion chamber, the cylinder output torqueremains the same irrespective of the secondary fuel injection. However,when the unburned fuel recirculates due to the secondary fuel injection,the cylinder output torque increases at the time when the secondary fuelis injected by an amount corresponding to the amount of the unburnedfuel that is recirculated (curve II in FIG. 16).

In this embodiment, when the output torque is increased by the amount ashown in FIG. 16 with a given main fuel injection amount due to thesecondary fuel injection while the EGR is being executed, therecirculating amount of the unburned fuel is estimated by presuming thatan increase in the amount of the main fuel injection (amount b in FIG.16) necessary for increasing the output torque by the amount a when theEGR is being effected without the secondary fuel injection, is equal tothe amount of the unburned fuel recirculating into the combustionchamber due to the secondary fuel injection.

That is, in this embodiment, curves corresponding to FIG. 16 areprepared, in advance, through experiment for every combination of theengine rotational speeds and the above-mentioned engine operating modes{circle around (1)} to {circle around (5)}, in order to calculate therecirculating amount of the unburned fuel (b in FIG. 16) in theirrespective main fuel injection amounts when the EGR is being effected.The values of unburned fuel amounts b are prepared as a numerical valuetable using the engine rotational speed NE and the main fuel injectionamount q_(INJ) as parameters for each of the operating modes, and arestored in the ROM of the ECU 30. When the engine is in operation, theECU calculates the amount b of the unburned fuel recirculating into thecombustion chamber when the EGR is being effected based on the enginerotational speed NE and the main fuel injection amount q_(INJ).

FIG. 17 is a flow chart illustrating the operation for correcting theamount of main fuel injection according to the embodiment. Thisoperation is executed at every predetermined crank rotation angle of theengine.

When the operation starts in FIG. 17, the engine load (degree ofaccelerator opening) ACCP and the rotational speed NE are read at a step1701, and the main fuel injection amount q_(INJ) is calculated at a step1703. Operations of the steps 1701 and 1703 are the same as those of thesteps 1003 and 1005 of FIG. 10.

Next, it is judged at a step 1705 whether the secondary fuel is nowbeing injected and at a step 1707, it is judged whether the EGR is beingeffected. When both the secondary fuel injection and the EGR are nowbeing effected, the unburned fuel recirculates into the combustionchamber due to the secondary fuel injection. At a step 1709, therefore,the amount b of unburned fuel that recirculates into the combustionchamber when the secondary fuel is injected is calculated from theabove-mentioned numerical value table prepared for each of the operatingmodes based on the relationship of FIG. 16, by using the main fuelinjection amount q_(INJ) calculated at the step 1703. Then, at a step1711, the main fuel injection amount q_(INJ) calculated at the step 1703is decreased by the recirculating amount b of the unburned fuel.

When either the secondary fuel injection or the EGR has not beeneffected at the steps 1705 and 1707, it is not probable that theunburned fuel recirculates into the combustion chamber. Therefore, thecorrections are not effected at the steps 1709 and 1711, and theoperation ends.

According to this embodiment, since the secondary fuel is injectedwithout being limited even when the EGR is being effected, both anincrease in the amount of NO_(x) emitted by the engine and a decrease inthe frequency for executing the operation for releasing NO_(x) from theNO_(x) occluding and reducing catalyst can be prevented.

According to the present invention as described above, the combustion inthe engine does not lose stability and the output torque does not changeeven when the ineffective fuel, which does not burn in the combustionchamber but is discharged together with the exhaust gas, is supplied tothe engine.

What is claimed is:
 1. A control system for an internal combustionengine comprising: an ineffective fuel-supply means for supplyingineffective fuel to the combustion chamber of an internal combustionengine; an EGR means for recirculating the exhaust gas from the engineinto the combustion chamber of the engine; an EGR limiting means forlimiting the exhaust gas recirculated by said EGR means when theineffective fuel is being supplied to the engine by said ineffectivefuel supply means; wherein said EGR means is equipped with an exhaustgas recirculation passage for connecting the exhaust passage of theengine to the intake passage of the engine and an EGR valve foradjusting the flow rate of the exhaust gas flowing through the exhaustgas recirculation passage, and said EGR limiting means limits therecirculation of said exhaust gas by controlling the opening degree ofsaid EGR valve to a value such that the EGR valve is not closed.
 2. Acontrol system for an internal combustion engine comprising: anineffective fuel-supply means for supplying ineffective fuel to thecombustion chamber of an internal combustion engine; an EGR means forrecirculating the exhaust gas from the engine into the combustionchamber of the engine; an EGR limiting means for limiting the exhaustgas recirculated by said EGR means when the ineffective fuel is beingsupplied to the engine by said ineffective fuel supply means; andwherein said internal combustion engine is equipped with a variablevalve timing device for varying the open-close timings of the intakevalves and the exhaust valves of the cylinders, and said EGR limitingmeans controls said variable valve timing means to change the open-closetimings of the intake valves or the exhaust valves to limit therecirculation of the exhaust gas to effect both exhaust gasrecirculation and secondary fuel injection simultaneously.
 3. A controlsystem for an internal combustion engine comprising: an ineffectivefuel-supply means for supplying ineffective fuel to the combustionchamber of an internal combustion engine; an EGR means for recirculatingthe exhaust gas from the engine into the combustion chamber of theengine; and an ineffective fuel limiting means for limiting the supplyof ineffective fuel by said ineffective fuel-supply means when theexhaust gas is recirculated by said EGR means.
 4. A control system foran internal combustion engine according to claim 3, wherein saidineffective fuel limiting means limits the ineffective fuel supplied bysaid ineffective fuel supply means when the flow rate of the exhaust gasrecirculated by said EGR means is larger than a predetermined amount. 5.An internal combustion engine comprising: a main fuel-supply means forsupplying, into said engine, the fuel that burns in the combustionchamber based on the operating conditions of the internal combustionengine; an ineffective fuel-supply means for supplying, into saidengine, the ineffective fuel to the combustion chamber of the engine; anEGR means for recirculating the exhaust gas from the engine into thecombustion chamber of the engine; and a correction means for estimatingthe amount of the ineffective fuel in the exhaust gas recirculated bysaid EGR means to correct the amount of fuel supplied to the engine bysaid main fuel-supply means based on the estimated amount.
 6. A controlsystem for an internal combustion engine comprising: an ineffectivefuel-supply means for supplying ineffective fuel to the combustionchamber of an internal combustion engine; an EGR means for recirculatingthe exhaust gas from the engine into the combustion chamber of theengine; an EGR limiting means for limiting the exhaust gas recirculatedby said EGR means when the ineffective fuel is being supplied to theengine by said ineffective fuel supply means; and a controller to effectboth exhaust gas recirculation and secondary fuel injectionsimultaneously and to discharge the ineffective fuel supplied by theineffective fuel supply means out of the cylinder during an exhauststroke of the engine.
 7. A control system for an internal combustionengine comprising: an ineffective fuel-supply means for supplyingineffective fuel to the combustion chamber of an internal combustionengine; an EGR means for recirculating the exhaust gas from the engineinto the combustion chamber of the engine; an EGR limiting means forlimiting the exhaust gas recirculated by said EGR means when theineffective fuel is being supplied to the engine by said ineffectivefuel supply means; and a controller to decrease an amount of main fuelinjected to accommodate any ineffective fuel which remains in thecylinder so that an amount of fuel that contributes to combustion in anext combustion cycle is in agreement with a target amount of main fuelinjected.